CARBONATE DEPOSIDONAL ENVIRONMENTS Edited by Peter A. Scholle, Don G. Bebout, Clyde H. Moore
Published by The American Association of Petroleum Geologists Tulsa, Oklahoma 74101, U.S.A.
x
Introduction
F
or more than 100 years geologists have been ex amining and describing modern sediments with an eye toward using characteristic features to aid in the interpretation of depositional settings of ancient strata. This field of interest developed par ticularly during the 1950s and 1960s with the creation of detailed models for modern carbonate deposition in Florida, the Bahamas, Cuba, the Per sian Gulf, Belize, Pacific atolls, the Great Barrier Reef and other areas. An understanding of the depositional environments of these modern models, coupled with increased understanding of diagenetic effects, has led to vastly improved interpreta tions of ancient limestones . Such models also improved the "predic tability" of many carbonate reservoir rocks. In spite of the great strides made in our knowledge about carbonate depositional environments, their characteristic features have never been synthesized in a single work. Although excellent textbooks exist which describe some aspects of the in terpretation of both ancient strata and modern sediments, systematic treatment of the entire subject is available only in the primary literature. This book is an attempt to bring
together this widely disseminated literature. The volume is specifically designed for use by the non specialist-the petroleum geologist or field geologist-who needs to use car bonate depositional environments in facies reconstructions and en vironmental interpretations. Yet it is hoped that the book will also serve as a valuable reference for the specialist or advanced graduate student . Toward that purpose, the book is extensively illustrated with color diagrams and photographs of sedimentary structures and facies assemblages. The text focuses on the recognition of depositional en vironments rather than on the hydrodynamic mechanisms of sedi ment movement. Assemblages of sedimentary structures and three dimensional facies geometry are ex amined as criteria for recognition of depositional environments in surface and subsurface sections. Although in dividual sedimentary structures generally are not diagnostic criteria, frequently the entire suite of struc tures found in a rock sequence can be used to recognize carbonate deposi tional environments, particularly when used along with vertical and lateral facies sequences, rock-body geometry, grain size analysis, and other techniques. All of these criteria are stressed in various chapters of this
book. In addition , the presence or absence of diagnostic organisms (or trace fossils) as well as the ecological characteristics of such faunal assemblages are discussed throughout the book. Early diagenetic processes are dealt with wherever they provide a clue to depositional settings. Rarely are diagenetic features absolute criteria for recognition of depositional en vironments but an understanding of such features may provide valuable additional evidence in such studies . In carbonate rocks, in particular, diagenetic processes start at the very moment of grain formation and deposition and the fabrics of diagenetic alteration (grain boring, leaching, cementation, etc.) are com monly environmentally sensitive. Finally, an attempt has been made to provide some perspective on large scale influences on carbonate deposi tion such as tectonic patterns, fluc tuations of sea level, variations of climate, evolutionary patterns of organisms, and patterns of ter rigenous sediment input. Carbonate sediments, because they are produced mainly by organisms (or through biochemical processes) , are particular ly susceptible to these influences. Although only a small number of dif ferent sedimentary environments ex ist, the factors mentioned above lead
Introduction
to a remarkable diversity of possible sedimentary patterns. The ultimate purpose of this book is to improve exploration for oil, gas, and mineral deposits. Understanding of depositional environments and ear ly diagenetic patterns are generally critical to the prediction of patterns of porosity and permeability. This is true both because depositional pat terns commonly control patterns of water movement and diagenesis in carbonate rocks, and because a con siderable amount of productive porosity in carbonate rocks is pre served from the depositional or early diagenetic setting . Thus, recognition of environments coupled with predic tion of trends can lead to important exploration advantages as well as im provements in secondary recovery strategy. The book is divided into individual chapters organized by environment of deposition. Although there is some variability between chapters due to the nature of available information for specific environments, most of the chapters are organized around a con sistent outline. The introduction defines and describes the facies in volved and provides a summary of the diagnostic criteria for the recogni tion of the environment . The second section provides information on rela tionships to adjacent facies, deposi-
tiona! setting, three-dimensional geometry of facies, and typical tec tonic relationships along with sedi ment composition, texture and consti tuent organisms. A third section outlines the major sedimentary struc tures characteristic of the environ ment and is heavily illustrated to show physical and biological struc tures of primary or early secondary origin . In some chapters, information is also provided on typical sediment textures and log responses. A fourth section gives information on economic considerations such as porosity potential of the overall en vironment and subenvironments, common diagenetic effects, and overall petroleum- or minerals trapping potential. Where available, ancient analogs are included at the end of each chapter . The book is organized with chapters in a specific order from non marine to deep-water settings . We have included environments in this book which are normally not con sidered to be particularly prospective for oil and gas. They have been in cluded for a number of reasons. First, the recognition of depositional environments is not really based strictly on the potential of those en vironments for producing oil and gas . We would like to provide as complete a framework for recognition of en-
xi
vironments as possible, and even units which are not necessarily pro ducers will still be involved in facies interpretation. Second, some of the environments which are not currently considered to be economically pro spective may contain, at some time in the future, "unconventional" reser voirs . We need only look back a few years to our views of lacustrine or deep-water sediments to see that en vironments which were previously completely written off as potential reservoirs have turned out, in some cases, to be very prolific ones . Lacustrine sediments are now major oil producers in the Uinta basin, in China, and in Brazil. Likewise, deep water limestones have turned out to be productive in several areas within the North Sea basin and in North America. This book describes only deposi tional environments of carbonate sediments. Equivalent sediments of clastic terrigenous origin have been discussed in AAPG Memoir 3 1 , a separate book with parallel format .
Peter A . Scholle Don G. Bebout Clyde H. Moore
2
Subaerial Exposure
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xposure surfaces occur on land and under the sea, but in this chapter we are con cerned only with subaerial exposure surfaces . More specifically we are concerned with the effects of subaerial exposure on carbonate se quences. Subaerial exposure surfaces are areas where upper bounding sur faces of sediment or rock show the effects of being exposed at the Earth's surface. In order to recognize fossil subaerial exposure surfaces they need to be exposed long enough to
allow subaerial diagenetic processes to modify or obliterate pre-existing fabrics. This will be recorded as a break in the sedimentary sequence. This usually means that significant periods of time have passed before exposed surfaces are buried by new deposits. What we mean by signifi cant periods of time is a relative con cept dependent on our limits of resolution and powers of observation. When considering absolutes of time we will offer only abstracts; our in tention here is not to quantify ab-
solutes of timing, duration or intensi ty of processes acting upon exposure surfaces, nor to examine critically the processes themselves. Rather we will document common and characteristic products of subaerial exposure, list criteria which aid in recognition of fossil subaerial exposure surfaces, and point out the significance and economic importance of subaerial ex posure surfaces in ancient carbonate sequences. Adhering to this outlined concep tual framework we can define a
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subaerial exposure surface as a distinct surface on land which in dicates: ( 1) non-deposition and com monly erosion; and (2) a break in the sedimentary sequence. Regardless of
cause or length, a subaerial exposure surface is a record of interruption in sedimentation. Other terms sometimes used to describe this condition include hiatus, diastem, break, disconformity,
zones (not to scale)
unconformity, hardground or discon tinuity surface. Even though all these terms refer to exposure surfaces, they may be submarine, subaerial, or both submarine and subaerial in origin. A
4
Subaerial Exposure
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profile with repeated changes in position of water table.
subaerial exposure surface is, by definition, an emersion or a land surface. Alteration Zone Associated with Subaerial Exposure
Below the subaerial surface itself there is an alteration or weathering zone whose thickness (ranging from a
few millimeters to hundreds of meters; fractions of an inch to hun dreds of feet) is controlled by a number of variables. The most im portant variables are climate, intensi ty and duration of subaerial diagenetic processes, position of the water table and its underlying freshwater phreatic lens, and the
petrofabric attributes of the attacked host sediment or rock. Because of these variables, a wide variety of features may develop within this alteration zone. Because these features help us identify, and are related genetically to, subaerial ex posure surfaces themselves, they are discussed in detail in later sections of
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this chapter. End-member Products of Subaerial Exposure
Basically, two end-member diagenetic carbonate facies, the karst facies and the edaphic or soil facies, are born as a result of subaerial ex posure. But because of the whims of preservation the soil facies is rarely preserved intact before the onset of subsequent erosion or sedimentation
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which will respectively remove the soil or incorporate, mix or dilute soil pro ducts with any new sediment influx. However, an important exception to this rule is the caliche or calcrete facies, whereby soil products become lithified prior to erosion or renewed sedimentation. Lithification of soil products by calcium carbonate forms indurated caliche facies which, like the karst facies, have a reasonable chance of being preserved in the rock
record. Fundamentals and Variations on a Theme
A subaerial exposure surface differs fundamentally from all other car bonate environments in one major aspect - it represents solely a diagenetic environment rather than a depositional one. All marine car bonate sediments are similar in one major aspect - once emergent , all
6
Subaerial Exposure
Figure 9-Co-existence of karst and caliche facies.
PERIODS
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should be greater in the former, are subjected to identical subaerial assuming equal chances of preserva diagenetic processes at their resulting tion in the rock record. Eolian and exposed surfaces. Given sufficient relative lowering of lacustrine carbonates are but slight variations on the same theme. Once sea level, subaerial exposure surfaces eolian carbonate sands are stabilized may exist within shallow-water or and lacustrine carbonates have dried deep-water marine carbonate se out, the subaerial diagenetic processes quences although their frequency
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wllich operate on these terrestrial car bonates will be the same as the pro cesses which operate on subaerially exposed marine carbonate sequences. In other words, subaerial exposure surfaces obey no rules with respect to environment of formation of the sub jected host carbonate. As long as
Subaerial Exposure
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Figure 11-Generalized sequences con· taining exposure surfaces based on known examples. (a) Karstified Mesozoic limestone reworked in Miocene coastal environment. Most of the karst profile is preserved, with clear evidence of prox· imity of soil cover, Barcelona, Spain. (b) Same exposure surface as in (a) but karst profile is not preserved; sequence is diagnostic of close proximity of karst and soil. (c) Caliche profile in Tertiary marine limestone reworked in Plio· Pleistocene coastal environments, Yucatan, Mexico. (d) Sequence probably contains subaerial exposure but no diagnostic evidence is found, except for relics of possible terra-rossa in deep joints. Same exposure surface as (a) and (b). (e) Mesozoic limestone, intensively calichified, followed by Paleocene palustrine sediments and alluvial fans, Barcelona, Spain. (f) Calichified over· bank deposits of Triassic alluvial fan complex, Barcelona, Spain. (g) Same ex· posure surface as (a), (b ) and {d), but without traces of subaerial exposure facies. Only coastal exposure facies are recorded.
Figure 12-Karst surface landforms. Dolinas (D), polje {P) on a relic karst plateau. Relic alteration zone {A) out· crops as cliff on steeply dipping unaltered substrate. We believe that some of these relic karst features developed in earlier tropical climates. Present-day Mediterranean type of karst processes only produce slight modifica· tions on the relic forms. Karst develop· ment from ?Lower Miocene {perhaps Lower Eocene?) to present on Cretaceous carbonates, Garraf Moun· tains, Barcelona, Spain.
there are: (1) subaerial conditions; (2) stabilization and non-deposition of sediment; and (3) sufficient time available for subaerial diagenetic pro cesses to affect the exposed host car bonate, then a subaerial exposure sur face and its underlying alteration zone will develop. Whether or not the original subaerial exposure surface and its underlying alteration zone will be preserved in the rock record depends largely on the presence or absence of any subsequent erosional phase.
Importance of Subaerial Exposure
As geologists, interested primarily in rocks, why should we concern ourselves with surfaces which repre sent missing pages in our story book of the Earth's history? There are at least three good reasons. Firstly, subaerial exposure surfaces provide important information when faced with the task of trying to decipher the geologic history of a region. In many instances, it is just as important to know what is missing from a sedimentary sequence as it is to know what has been preserved. For example, when subaerial exposure
7
surfaces can be identified within marine carbonates, important deduc tions can be made regarding periods of regression or upbuilding of sedi ment packages above sea level. Secondly, subaerial exposure surfaces can be extremely useful horizon markers for outcrop or core correla tions. Thirdly, and perhaps of greatest economic importance, subaerial exposure surfaces are sites where valuable natural resources can be concentrated, including: (I) oil, gas or water traps in which the seal ing unit overlies the reservoir rock below the subaerial surface; and (2)
8
Subaerial Exposure
Figure 13-Tropical karst landforms. Tower and conical karst developed since the Pleistocene on Cretaceous car bonates. North side of Mayan Moun tains. Belize.
Figure 14-Dolinas. Aerial view
of
lowland plain with water-filled dolinas developed on Tertiary carbonates. These dolinas are believed to have formed in pre-Holocene temperate climates (Purdy, 1974}. Location about 60 km (37 mi) north of Belize.
various accumulations and deposits of certain me tal s (lead, zinc, uranium, bauxite) related to soil processes or precipitation in karst cavities.
SETTING Relationship to Lateral Facies
Subaerial exposure surfaces grade
Exposure surfaces in general may be highly irregular, or smooth and planar. Thus, exposure surfaces are
This category includes submarine hardgrounds with or without their manganese, phosphatic or glauconite
characterized by their topography and
crusts and will not be considered fur
relative position to sea level. They may be developed on sediment or rock (Fig. 2).
ther in this chapter.
Submarine exposure surfaces
-
Carbonate sediments or rocks are ex
laterally seaward into coastal and
posed on the sea floor below the
submarine exposure surfaces (Fig. 1).
lower limit of noticeable wave action.
Coastal exposure surfaces
-
Sediments or rocks are exposed in the p eritida l environment above the lower limit of wave action and below the highest lim it of the marine vadose salt spray zone. Some of the most com-
Subaerial Exposure
9
Figure 15-Fossilized rundkarren and solution pipe. Terra-rossa type soils fill depres sions in karst landforms. Pleistocene, Bermuda.
Figure 16-Fossilized rundkarren. Intra· Triassic karst development on Lower Muschelkalk carbonates ove r l ain by Up· per Muschelkalk dolostones. Middle Muschelkalk red beds are represented as a condensed sequence of red soils covering the karst surface. Barcelona . Spain .
mon features of the coastal environ ment are shown in Figure 2a for rocky shores and in Figure 2b for sediment shores . Very irregular sur faces produced by wave action and bioerosion are characteristic of the rocky shore environment. Stabiliza tion by plants is important in the sediment shore environment . Of special interest for our purposes is the upper part of the coastal en-
vironment with rock pools infested by blue-green algae (Schneider, 1976), pelagosite crusts (Purser and Loreau, 1 973) and the dissolution forms refer red to as coastal marine karst . The coastal environment is of interest because of its juxtaposition to the subaerial meteoric environment and its high preservation potential in the geologic record. With changes of sea level, complicated patterns of
superimposed coastal and subaerial processes may act on the same ex posure surface. Renewed sedimenta tion will preserve this polygenetic surface.
Subaerial exposure surfaces
-
Sediments and rocks are exposed to the atmosphere landward of any marine influence. Lake bottoms and deflation flats are excluded from this discussion. First evidence of subaerial
10
Subaerial Exposure
Figure 17-Sink-hole. Chaotic mass of red-stained brecciated blocks and sediments with marine Carboniferous fossils, developed as sink-llole fill in gray gently dipping Middle Ordovician
l imestones . Western Newfoundland, Canada.
exposure can be established by the appearance of endolithic lichens which represent the first stage of col onization and subsequent community succession in subaerial environments. This limit, established on the basis of lichen colonization, is not generally recognizable in the fossil record. A more practical limit for the geologist is the first appearance of karst and caliche diagenetic facies. Succession and Modification of Exposure Surfaces
Sea level oscillations and variations in tectonic activity will produce im portant modifications and successions in exposure surfaces. With a relative sea level rise, areas subaerially ex posed will eventually be overlain by a marine sediment (Fig. 3). During a relative fall of sea level, subaerial ex posure surfaces will develop superim posed on the coastal and submarine exposure surfaces (Fig. 3) and even tually be overlain by a continental se quence (eolian sands, alluvial fans, lacustrine and palustrine sediments, fluvial sediments, porous and dense plant tufas). The last exposure sur face prior to renewed sedimentation is the only one preserved on most occa-
sions; it is of no coincidence that the best evidence of subaerial exposure is found generally at the base of a se quence of continental deposits. The profile of exposure surfaces may also change with time without relative sea level oscillations. Many exposure surfaces are accompanied by destructive processes and the general trend is toward lowering of elevation and planation of surfaces. Exposure surfaces may also be modified tec tonically (fracturing, orogenic and epeirogenic uplifts) with the conse quent reactivation of erosional and depositional processes . Climate exerts an important con trol, both in time and space, on mor phology and profile development of subaerial exposure surfaces . Spatially, a single extensive subaerial exposure surface can be subjected to different climatic regimes (for example, from a dry warm low plain to a wet cool mountainous region). With time, climatic belts may shift , subjecting a given point on a subaerial exposure surface to a new set (or new intensity) of climate-sensitive processes . All these controls, position with respect to sea level, tectonic activity and climate, together with the inces-
sant evolution of diagenetic processes, will modify the topographic expres sion of exposure surfaces. Subaerial Exposure Facies
Although caliche can develop in rock or sediment of any composition and karst may develop in evaporite bodies in addition to carbonate ter rains, only karst and caliche developed in carbonate hosts are rele vant to this discussion. Generalities Two major diagenetic carbonate facies are pro duced as a consequence of subaerial exposure, the karst facies and the caliche facies . Both diagenetic facies involve mobility of calcium car bonate . In the karst facies there is a net loss of calcium carbonate because of dissolution and removal of CaC03 solutions (supplemented by mechanical erosion). In the caliche facies there is a zero balance (local dissolution and reprecipitation of CaC03 without external sources) or a net gain (addition of CaC03) from elsewhere. Movement of CaC03bearing solutions occurs both in the meteoric vadose and meteoric phreatic zones. These hydrologic zones are depicted in Figure 4, show-
Subaerial Exposure
11
Figure 18-Lapies. Irregular, jagged sur face truncates Upper Miocene reef limes1one overlain by Pliocene beach complex. Red-stained silt, probably deriv· ed from terra-rossa soils, covers karst surface. Mallorca, Spain.
ing the meteoric vadose zone sub divided into an upper dissolution zone of infiltration and a lower per colation zone Q''f downward move ment with negligible dissolution of calcium carbonate. Similarly the meteoric phreatic zone can be sub divided into an upper active lenticular zone (Jakucs, 1 977) and a lower inac tive deep zone (imbibition zone) which grades downward into connate or formation waters . In the zone of infiltration, waters are active and may either ( 1 ) dissolve calcium carbonate because of ag gressive atmospheric C02 and/or high levels of biogenic C02 derived from overlying soil cover, or (2) precipitate calcium carbonate if C02 levels are lowered by degassing or plant uptake. By the time water reaches the underlying percolation zone it is at CaC03-H2C03 equilibrium and dissolution or precipitation of calcium carbonate is minimal. In contrast, in the upper most part of the phreatic zone just below the water table, water move ment is essentially horizontal and CaC03 is either removed by dissolu tion and mechanical erosion or precipitated. Continuing downward to the lenticular zone, whose position is determined by an underlying im permeable horizon or by the base
level of erosion, direction of water movement is controlled by hydraulic pressure rather than direct gravity. Limestone in this zone may undergo dissolution because of increased hydrostatic pressure and mixing cor rosion, or precipitation may take place if mixed waters become super saturated with respect to calcium car bonate. Water below the lenticular zone, even though under hydrostatic pressure, does not play a role in the hydrographic cycle of the overlying lenticular zone; it therefore attains equilibrium with its surroundings and becomes stagnant . This inactive zone grades downward through a transi tional zone into connate waters whose properties and effects on calcium car bonate are poorly understood or unknown. Karst Facies
The term karst has been used to designate specific landforms (in cluding subterranean landforms) and a geographic region characterized by these landforms. Modern develop ments (Thrailkill, 1 968, 1 97 1 ; Sweeting, 1973; Jakucs, 1977) have stressed the concept of karst as a result of a complex set of processes (climatic, tectonic, edaphic, hydro logic, petrologic) with different evolu tionary stages. On this basis, the
following definition is more useful to geologists: "Karst is a diagenetic facies, an overprint in subaerially ex posed carbonate bodies, produced and controlled by dissolution and migration of calcium carbonate in meteoric waters, occurring in a wide variety of climatic and tectonic set tings, and generating a recognizable landscape." Karst facies represent a net loss of calcium carbonate, although in some stages of karst evolution or in some parts of the pro file, it is possible to have equilibrium or gain in the carbonate budget . A descriptive characterization of karst should be based on: (I) surface land forms (lapies, dolinas, poljes); (2) subterranean landforms (varieties of pores, caves, vugs, pipes); (3) speleothems (stalactites, stalagmites, flowstones, rimstones, globulites, cave pearls, lily pads, helictites, moon-milk); and (4) collapse struc tures due to removal of underlying carbonate. Mineralogy of the karst facies is dominantly low magnesian calcite (LMC), although caves in dolostones show a wider variability in mineral ogy with local deposits of aragonite, hydromagnesite, dolomite and nes quehonite. In this chapter we will refer generally to LMC deposits because they are more abundant and
12
Subaerial Exposure
Figure 19-Collapsed roof of cavern. Cavern formed in Upper Miocene reef limestone during time of formation of subaerial exposure surface of Figure 18. Mallorca, Spain.
cave deposits of other mineralogies show roughly the same gross morphologies. Karst Profile - Karst presents a generalized facies pattern that can be summarized in a profile (Fig. 5) as follows: ( 1 ) Infiltration (upper vadose) zone, characterized by surface landforms, with or without a developed soil cover (exposed carbonate usually has a lichen or algal cover, a protosoil) and by production of dominantly ver tical caves (pecina, Figs. 20, 26). Most of the speleothems of this zone are the result of intense corrosion and degradation of carbonate host walls. These speleothems are fine grained deposits (moon-milk) made of ran dom needle fibers of low magnesian calcite (LMC) and some types of globulites (popcorn) growing in degraded carbonate hosts. The final product is a characteristic accumula tion in walls and cavern floors of white, fine-grained deposits of chalky or friable consistency, abundantly colonized by fungi and bacteria. The dominant processes in this zone are physicochemical dissolution and bio logical corrosion related to intense organic activity. Collapse breccia deposits can also be very abundant (Fig. 25) in zones up to 20m (66 ft) thick in many European and North
American karst profiles. (2) Percolation (lower vadose) zone, characterized by net vertical water movement through pre-existing permeability pathways. In percolation zones dominated by vadose water seepage there is little dissolution, but localized areas of vadose flow (under lying major sinkholes, thick soil covers, open fractures) show active dissolution and hydraulic erosion. As a facies this zone is poorly defined, oscillating between 0 to 200 m (0 to 660ft) thick, but the lower part near the phreatic water table is where spe leothem formation is more abundant and more varied than elsewhere in the karst profile. The presence of a capil lary fringe (up to 2 m or 6.6 ft above the water table) is likely related to this intense carbonate precipitation. (3) Lenticular (upper phreatic) zone, characterized by intense forma tion of subhorizontal caves Uama) by hydraulic erosion and dissolution as a result of mixing corrosion and in creasing hydrostatic pressure. Subver tical caves can also be formed but they are subsidiary to horizontal drainage patterns. Most karst cavern porosity is produced in this zone, especially just below the water table (Thrailkill, 1968) in allogenic karsts. Speleothem formation in this zone can also be important in senile stages
of evolution, mainly at and a few centimeters below the water table (floating carbonate flakes or rafts, isopachous coatings, cave pearls, lily pads). Collapse breccias and water laid deposits are very abundant local ly. In karst facies it is common to have perched water tables at various levels, thus introducing more com plexity to the profile. The lower limit of the karst profile is difficult to establish. Lenticular zones can be up to 100m (330 ft) thick or more. The increase in hydrostatic pressure at the base of the lenticular zone may favor dissolution but, at a deeper level, waters will become stagnant and a diagenetic overprint related to subaerial ex posure is no longer recognizable. On the other hand, the lenticular zone could be the ideal setting for per vasive phreatic cementation, a process accepted by many carbonate petrologists. Intergranular cements and speleothem deposits of the phreatic lenticular zone may be dif ferent products of the same cementa tion process; the differences being controlled by two factors - pore geometry and water flow pattern. Evolution - The karst profile of Figure 5 could represent a mature stage of evolution of a karst facies. A dynamic view of the karst facies of-
Subaerial Exposure
13
Figure 20-Speleothem. Plan view of vertical solution pipe in Upper Miocene reef dolostone. Solution pipe wall is delineated by iron-rich crust which is overlain by banded calcite and minor aragonite flowstones and globulite (pop corn). Karst event is same as in Figures 18 and 19, but located here In Allcante,
Spain.
Figure 21-Kamenitza. Recent solution rock pools developed on Tertiary car bonates in tropical climate, Yucatan, Mexico.
fers more complexity. To start with, in an absolutely massive, pure and homogeneous carbonate host, early stages of karst evolution would be confined to surface landforms (for example the experimental studies of Purdy, 1974). Heterogeneities such as fractures, joints, bedding planes, im purities in the carbonate host and vegetation favor the development of subterranean landforms. With a stable phreatic base level, the evolu tion of the karst profile (Fig. 6) im plies the lowering of the surface and the downward shift of the infiltration zone. The surface evolves into com plex landforms such as dolinas, uvalas, poljes and karst valleys and plains (Fig. 12). Insoluble residues of the carbonate host play a major role in karst facies evolution; they are the basic contributors to the formation of soil cover (with the consequent in crease of biogenic C02 taken up part ly by meteoric waters). Deposition of insoluble residues in subterranean pores (Fig. 33) and conduits controls the hydrologic patterns of karst. With the exception of tropical climates, the tendency is toward a limestone pla teau where the infiltration and len ticular zones merge. The lenticular zone is drastically reduced because of clogging by insoluble residues. Tropical karst, on the other hand,
differs from this model. Accentuation of surface morphology, because of the relatively higher soil permeability and aggressiveness of waters in the lower depressed areas occupied by soils, produces conical and tower karst (Figs. 6, 13). Profiles of karst facies are usually complicated by repeated changes in position of the phreatic water level (variations in tectonic and climatic
setting, sea level oscillations). For ex ample, products of the phreatic zone can be subjected to processes in the upper vadose zone and become coated, corroded, dissolved or remodelled. Similarly, former vadose products can be drowned and modified in the phreatic zone (Fig. 6). In general, the karst profile is more active during periods following oro genic episodes because of: (1) the
14
Subae rial E><posure
F i g u re 22-Lapies. Enlarged open joi nt (kluftkarren) and root lapies modified by i nc ipient ri l l en kar ren . Kl uftka r ren and root lapies are possibly rel ic karst forms of wetter climate than present day, sem iarid Mediterranean climate. M iocene limestones , Almeria , Spain.
creation of new joints and fractures in the host rock which allows percola tion of more meteoric water, (2) the increase in topographic irregularities, with deepening of the water table and easy removal of soils on unstable slopes; and (3) the formation of topo graphic highs which can control local climatic patterns favoring orographic rains. The repetition of these changes strongly alters the host rock , with recognition of these multicyclic episodes and modifications extremely difficult but possible with careful geomorphological, petrological and paleontological studies . In the fossil record, only parts of the karst facies and profiles are likely to be preserved and the study of karst zones and their inferred evolution will be generally impossible. The profile summarized here cor responds to authigenic karst (halo karst) where meteoric waters in the profile are collected in the infiltration zone of the same profile. Other possi ble types of profile occur where meteoric waters arrive into the profile from adjacent or suprajacent non karstic terrains (allogenic karst of Jakucs, 1977). Most karst profiles in nature are combinations of authigenic and allogenic types. Allogenic karst is very important in mature and senile stages of authigenic karst, typically
under thick soil cover or below non carbonate formations. The allogenic karst facies results from valley sculp ture by linear stream bed erosion and differs from authigenic karst mainly because in allogenic karst: ( l ) the drainage pattern has a marked polari ty which clearly resembles an under ground river, (2) deposition of speleo thems and cave sediments is more abundant , and (3) scour caverns, typically with flat roofs, are abun dant. For geologists working in the fossil record, the distinction between the authigenic and allogenic karst is extremely difficult; our discussion will refer to both types together. Another important aspect of karst evolution is the intimate relationship with the soil cover . The definition of the term karst does not include soil forming processes, although a variety of soils are common on different types of karst . These soils are an im portant control and consequence of karstification. As a control , soils are sites of biogenic C02 production and meteoric water storage . As a conse quence, soils are formed from insolu ble residues as a by-product of karsti fication. In general, early stages of karst evolution show poorly developed soil cover while later stages usually imply thicker and more exten sive soil cover. The climatic setting,
together with lithological and topo graphic patterns of the carbonate host, are major factors which control development of the soil cover . Residual soil deposits, such as terra-rossa and laterites , may be char acteristic of specific climatic types of karst (Mediterranean, tropical) and can be recognized within fossil car bonate sequences (Figs . 18, 27, 36, 37) . The soil cover of karst may also evolve in the direction of carbonate rich soils of the caliche type (Fig. 3 8) , particularly in mature and late stages of karst evolution . Because of their high lithification potential, caliche soils are one of the most useful cri teria for recognition of fossil sub aerial exposure surfaces and deserve careful attention . Caliche Facies
Caliche (calcrete) is commonly defined as a fine-grained , chalky to well-cemented, low magnesian calcite deposit that formed as a soil in or on pre-existing sediments, soils or rocks in semiarid environments (Bretz and Horberg , 1949; Brown, 1 956; Swine ford et al, 1 958; Durand, 1 963 ; Blank and Tynes, 1 965; Gile et al, 1 966; Ruellan, 1 967; Aristarain , 1 970; Reeves, 1970; James, 1 972; Esteban, 1 972, 1 974; Read , 1974). This essen tially genetic definition is of limited
Subaerial Exposure
15
Figu re 23-Rillenkarren and small kamenitza. Modern surface karst features developed on fine-grained limestone of Cretaceous age. Castello de Ia Plana, Spain.
.
.
.
Figure 24-Solution p i pe and caliche
profile Solution pipe, developed in redbrown Pleistocene calcareous silts lined with laminar m icritic crust. Overl in by
use to geologists lookmg for cal iche in the fossil recor� . I.t is ne:e�s.ary to have a more descnpt1ve defm1t1on which includes regional setting, lithologies and their sequences, structures, textures and fabrics. A descrip tive definition has been proposed by Esteban ( 1 976) and is adapted here, with some modifications, as follows: "Caliche is a vertically zoned , sub horizontal to horizontal carbonate deposit, developed normally with four rock types: (1) massive-chalky, (2) nodular-crumbly, (3) platy or sheet like, and (4) compact crust or hard pan. The position and development of these rock types in a vertical se quence (profile) and laterally is highly variable . The only rather consistent relation is that the massive-chalky rock grades downward into the original rock or sediment through a transition zone, with strong evidence for both in-place alteration and replacement of the original rocks or sediment . Colors are commonly white and light brown, but red and black may be important . The predominant caliche fabric is a clotted, peloidal micrite with microspar channels and cracks . Accessory fabrics are rhizo liths, glaebules (pisoliths, ooliths, nodules, peloids), poorly laminated micrite and karst products . Microspar areas usually show evidence of
�
hardpan and nodular caliche of later Pleistocene age. lbiza, Spain.
replacement of relic grains and of other primary and earlier diagenetic microfabrics . " Caliche is an informal term used by geologists. Probably any carbonate-rich soil, with CaC03 con centrated in the form of glaebules, can be fossilized to form different types of caliche rocks. Caliche Profiles An idealized caliche profile is shown in Figure 7, and though only one of many possi ble variations it is the most common of modern western Mediterranean and Texas caliches. Other common profiles are shown in Figure 8. The term caliche profile refers to the com plete vertical succession of morpho logically distinct layers or horizons . Boundaries between horizons tend to show gradual transitions rather than abrupt changes . The main features of each horizon are summarized in Figure 7 and Table 1 an d described as follows: Hardpan - Being well indurated and lacking visible porosity, the hard pan is generally more resistant to weathering than underlying horizons and, thus, commonly stands out as a -
prominent feature. Vertical thick nesses vary from l mm (0.04 in.) laminar layers (Fig. 75) to massive horizons, 1 . 5 m (5 ft) thick. The horizon is made up dominantly of well cemented microcrystalline or cryptocrystalline calcite. The thicker hardpans commonly show evidence of fracturing, non-tectonic brecciation (Figs . 4 1 , 44, 60, 63, 68, 72), dissolu tion and recementation, and may con tain glaebules (pisoliths) and rhizo liths . Colors are generally white or cream , although pale orange to brown are not uncommon . The hard pan may be macroscopically struc tureless or massive, laminated (Fig . 58), brecciated (Fig . 60) or nodular (Fig. 74). Platy caliche - Platy caliche in variably occurs immediately below the hardpan horizon (Fig. 43) or, in pro files lacking a hardpan, occurs as the uppermost calcareous horizon or below a more recent soil cover. Platy caliche is distinguished from hardpan caliche by its horizontal to subhor izontal, platy, wavy or thinly bedded habit (Fig . 44), its planar fracture
16
Subaerial Exposure
Figure 25-Collapse breccia. Part of fill in solution pipe formed in Pleistocene carbonates. Angular blocks of carbonate t1ost coated with flowstone. Flowstone forms a geopetal fill within interblock porosi ty. Jamaica.
porosity and greater friability, and the abundance of alveolar textures, rhizoliths and needle fiber fabrics. Recognizable transitional stages sug gest that hardpans represent advanced stages of platy caliche development . Maximum and average thicknesses o f platy caliche horizons are greater than those of hardpans, the maximum thickness recorded for platy caliche from Spain being 3 . 1 m ( 10 ft) . Platy caliche generally grades downward in to nodular caliche. Nodular caliche - Nodular caliche is made up of glaebules which consist of discrete, powdery to indurated concentrations of calcium carbonate embedded in a less carbonate-rich matrix (Figs. 39, 40, 47). Individual glaebules range from silt-sized to pebble-sized particles . They may be spherical or subspherical , irregular or cylindrical in shape. Most cylindrical forms are vertically elongate (Fig. 43); some, however, show branching patterns . Glaebules occur either as isolated particles or as coalesced masses . In addition to greater concen trations of calcium carbonate and greater cohesion, glaebules usually can be distinguished from the sur-
Figure 26-Speleothem Laminar flowstone of Pleistocene age. (Field view of flowstone and host rock shown in Fig. 29) lbiza. Spain Thin section. plane polarized light. Scale bar = 1 0 mm.
rounding matrix by color differences. Glaebules are commonly white to cream in color, whereas the matrix tends to be red or red-brown (Fig. 40) because of higher concentrations of acid-insoluble residues such as layer lattice minerals and ferric hydroxides . However, some glaebules, especially sand-sized glaebules with internal concentric fabrics, are dark red brown whereas the matrix is pale brown to cream. Iron salt impregna tion within these darker glaebules is usually responsible for this color dif ference. Nodular horizons invariably show diffuse upper and lower bound aries . In many profiles, the boundary between nodular and chalky-powdery horizons is so indistinct that this "boundary" can commonly be treated as a separated horizon, that is, the nodular-chalky horizon. Chalky caliche - The chalky caliche is characterized by white to cream, unconsolidated silt-sized calcite grains. Cementation between grains is absent so that the material has the consistency of a powder. This horizon tends to be homogeneous structurally and texturally, although scattered, isolated glaebules are local-
ly present (Figs. 39, 46) . Areas around plant root systems appear to be favored sites for CaC03 ac cumulation, and this phenomenon gives rise to incipient nodular development. Maximum thicknesses of this horizon rarely exceed 1 m (3 . 3 ft). Chalky horizons are poorly developed or absent in most profiles where the host material has high in terparticle porosity (for example, eolianites). The chalky horizon generally grades upward into nodular caliche and downward into the transi tional horizon . Transitional horizon - The term transitional horizon refers to the zone between unaltered host material and overlying caliche horizons that lack macroscopically discernible features inherited from the host material . The transitional horizon itself contains macroscopically discernible evidence of in-place alteration and partial replacement of the original host material (Fig . 5 1) . This evidence in cludes: ( 1 ) relic sedimentary structures such as bedding (Fig. 46), (2) in-place relic fossils embedded in otherwise calichified host material, (3) in-place relic siliciclastic grains with distribu-
Subaerial Exposure
17
Fi gure 27-Cavern sediment fill. Light· cream areas are host carbonate (Cretaceous). Horizontally laminated, light brown areas are lateritic, granule to sand size sediments (younger Cretaceous) filling paleokarst cavities (D' Argenio, 1967). These fossil karst deposits are micrite-rich and are related to overlying lateritic soils, Southern Ap pennines. Italy.
Figure 28-Cave pearls and laminated cavern sediment. Same locality as in Figure 27. Cave pearls are also micrite rich and occur in layers with characteristic coarsening upwa rd grain size arrangements.
tion patterns inherited from the host material, and (4) relic mineral veins traceable from the underlying host material into the caliche profile without deviation or disruption. In profiles developed within bedded sedimentary deposits, dipping beds of unaltered host material can be traced upward into transitional and chalky horizons. Alteration takes place pre ferentially along bedding and joint
planes (Fig. 68) . These planes of ac cess allow water to move through host material more easily and are, thus, sites susceptible to diagenetic alteration and to penetration by roots . This horizon is essentially an in-place weathered zone consisting of partially degraded host material, making it difficult to fix the lower boundary . The transitional horizon may grade into the upper vadose zone
of karst and may show thicknesses of several meters (feet). In other profiles the transition horizon can be minimal or apparently absent. Host material - The host material may be of any composition, texture, age and origin. The only significant factor of host material that influences caliche development is its mechanical stability; development of a caliche profile requires a stable substrate suf-
18
Subaerial Exposure
Figure 29-Speloethem and breccia. Brecciated (karst collapse breccia?) and recemented (and calichified) Kim meridgian dolostone and laminar flowstone of Pleistocene age. Thin sec tion photomicrograph of flowstone shown in Figure 26. lbiza, Spain.
Figure 30-Endolithic lichens. Rock sur
face (Cretaceous limestone) shows an early development of rillenkarren which is smoothed out by endolithic lichens. Sharp, irregular contacts (L) between dif ferent lichen communities correspond to areas of intense frugal penetration into the rock. Garraf Mountains, Barcelona, Spain.
ficiently long for pedogenetic and dia genetic processes to operate. Even so , other factors of the host material such as permeability and calcium car bonate content can influence the rate of caliche development . The host material is distinguished from its overlying caliche profile by the ab sence of typical features which char acterize caliche. Original structures, textures and fabrics of the host
material are clearly recognizable; they have not been modified or obliterated by subsequent calichification in con trast to the transitional, chalky, nodular, platy and hardpan horizons which do show such alterations with increasing intensity upward and away from the host material . Evolution Unconsolidated caliches may be active, relic or fossil CaC0 3-rich soils. Indurated caliches -
are lithified fossil CaC03 -rich soils. Soils are products of weathering and also the natural medium in which plants grow. The evolution, from a subaerially exposed marine carbonate to an indurated caliche rock, can be delineated simplistically into five stages: Stage I: Preparation of host material; weathering. Mechanical , physicochemical and biological dis-
Subaerial Exposure
19
F1gure 31-Endolithic lichens. Profile of modern endolithic lichen colonization in Jurassic dolostone. Green cavities below the surface a re the a lg al components of t he lichen symbiosis (gonidial zone, G) Blac k filaments correspond to the fungal hyphae (medulla zone. M). The lower part contains unaltered host rock. Spheroidal depressions and cracks in the surface are made by tormer reproductive bodies of the lichen, Mallor ca. Spain. Photograph courtesy of L. Pomar. Thin section. plane polarized ligtll. Sca!e bar
integration generate a regolith or accumulation of weathered detritus. Soil development implies rates of ac cumulation exceeding rates of removal. Stage 2: Soil development; pedo genesis. Unconsolidated sediment or weathered detritus is developed into a soil by changes produced by the ac tion of organisms and by movement of water through the sediment. Stage 3: Accumulation of calcium carbonate and differentiation into horizons. In the early stages of ca liche development, the profile is com posed of weathered materials with high porosities and permeabilities. Vertical movements of meteoric vadose water can take place relatively easily, and insufficient water is retain ed to supply the requirements of the vegetation. Some plants have to ex tend taproots vertically downward to the proximity of local water tables. Roots extend downward into frac tures and joints within host material , modifying original structures and contributing to the disintegration of the substrate. Biological and physicochemical alterations of the host material culminate in the forma tion of a transitional horizon. Precipitation of calcium carbonate, without significant cementation because of mechanical and biochem-
ical instability of the profile, forms most of the chalky horizon . Pedotur bation (physical, chemical and biological disturbance of soil materials) precludes the formation of indurated layers. As the accumulation of calcium carbonates increases, porosity and permeability of the profile decrease. Biological constituents of the soil may become calcified, thus forming bio genetic carbonate structures such as rhizoliths, calcified filaments, calcified fecal pellets, calcified coc coons and Microcodium aggregates. Wetting and drying of the soil favors development of shrinkage cracks and, later, precipitation of CaC03 within voids. In earlier stages of profile development , vertical water move ments and vertical taproots tend to form vertically oriented elongate car bonate nodules. But at some point of profile development, it becomes easier for soil water to move horizontally rather than vertically. By this stage most plants form lateral root systems, their corresponding rhizoliths form the bulk of the platy caliche horizon . Vertical rhizoliths in lower parts of the profile tend to be large (5 to 20 em; 2 to 8 in) and isolated ; platy horizon rhizoliths are smaller (0.5 to 2 mm; 0.02 to 0.08 in), extremely abundant, branching and horizontally
=
0.5 mm.
oriented . Development of the platy caliche horizon from the nodular horizon may be a reflection of plant succession in a developing soil. Stage 4: Lithification , cementation and fossilization. As accumulation of calcium carbonate increases, a point will be reached when soil organisms can no longer maintain viability. The intensity of soil-forming processes diminish and eventually cease to be important . Diagenetic processes, mainly cementation by low magnesian calcite, lead to the lithification and fossilization of the soil profile and formation of a hardpan . Stage 5 : Reworking, brecciation, weathering (new Stage 1). The lithified caliche profile, if it remains at the land surface, is subjected to further processes which will alter or destroy the profile. Lower plant (lichens, algae, fungi , bacteria) ac tivities will form a protosoil, a pioneer stage in plant community suc cession. Eventually the developing soil profile is able to support higher plants. The root systems of these plants penetrate, dissolve and fracture the indurated hardpan. Disturbance of the caliche profile by vegetation may form tepee structures and rhizo breccias. Further pedoturbation , with carbonate dissolution and reprecipita tion, leads to the formation of a
20
Subaerial Exposure
Figure 32-Fossilized lichen-colonized surface. Iron stained fungal hyphae of endolithic lichen overlain
by
dark iron
stainl'!d and org an ic-rich layer. Lichensynthesized calcite prism shown on right both within and hP.Iow organic-rich layl'!r. Spain.
pi�1i������i�
n:1��l�����������· L�-���)�:
reworked, recemented, breccia conglomeratic caliche hardpan. To complicate matters, the reworked recemented profile may be later sub jected to a karst over-print . Spatial and Time Relationships Between Karst and Caliche Facies
Processes operating in both karst and caliche diagenetic environments are not mutually exclusive. Different stages of karst and caliche evolution may co-exist at any one time and overlap in any one area (Fig . 9). In addition, with time the caliche facies may be subjected to karstification and renewed soil formation and, like wise, karstified surfaces may become calichified. Caliche and karst facies are dynamic systems involving com plex processes controlled by many factors and including climatic varia tions, organic activity, and character istics of the host substrate. Moreover , both facies influence their own evolution. Some important differences be tween the karst and caliche facies do exist, however . One difference is that caliche is attributed to a specific climatic regimen (Goudie, 1973), namely a semiarid zone, whereas karst facies develop in all climates with water . In addition , while the caliche facies is restricted essentially
to the surface and near-surface subaerial vadose environment , the karst facies is a three-dimensional unit which occurs both at the surface of the Earth and extends downward through the vadose and into the meteoric phreatic environment . The karst and caliche facies thus show dif ferences in magnitude and hydrologic regimen. Nevertheless, as will be seen later, some caliche products are similar to those which form in the up per vadose zone of the karst facies. The subaerial vadose zone is the loca tion where lithosphere, atmosphere and biosphere interact in a complex manner to produce features common to both karst and caliche facies . Subaerial Exposure Surfaces in the Geologic Record
Subaerial exposure surfaces from Silurian-Devonian times onward are well documented (see Bibliography). Earlier occurrences are scanty (Geldsetzer , 1 976) and present more problems of interpretation. The weathering processes inferred above are heavily dependent on the presence and evolution of higher plants, main ly because of the production of biogenic C02 which increases the ag gressiveness of meteoric waters. Before the Silurian, ? caliche and ? karst processes would necessarily be
governed by C02 obtained from lichens, algae, fungi and, possibly , from higher concentrations of at mospheric C02. We suspect that karst and caliche facies before the Silurian, without higher plant in fluence, had di fferent features than the ones described in this paper . However , available references are not detailed enough for conclusive results in this respect . It is well known that some intervals or episodes of the geologic record contain, simultaneously world-wide, abundant mineral deposits of lead zinc and bauxites in karst terrains (Bernard, 1 976; Geldsetzer, 1 976; Padalino et al , 1 976; Valeton , 1 97 2 for general references). The references to possible fossil caliches (see Bibliography) also occur at times of mineralized karst ; the same appears to be true for oil reservoirs in karst caverns. Some of these periods of generalized subaerial exposure have been frequently reported in the literature, either because of their economic importance or because of their intensive and widespread development, or both. As shown in Figure 10, these intervals of more pronounced or more studied subaerial exposure follow (by 10 to 1 5 m .y.) major stages or milestones in plant domination. This apparent coin-
Subaerial Exposure
21
Figure 33-Karst sediment fill. View
looking downward onto upper surface
(parallel to bedding). Reddish brown
terra-rossa fill between coral heads of
Pleistocene limestone, Florida.
cidence can be explained in two inter related ways: ( l ) major episodes of generalized subaerial exposure may favor mass extinction of established terrestrial plants and expansion of new types, and (2) onset of domina tion stages of terrestrial plants may be reflected in the type and intensity of subaerial exposure facies . During extended periods of subaerial ex posure, land colonized by expanding new types of plants could produce most intense diagenetic modifications in the host rocks, resulting in easily recognizable subaerial exposure facies and in most intense accumulation of mineral deposits. As should be expected, major subaerial exposure facies occur during the global cycles of sea level fall (Fig. 10) established by Vail and others ( 1 977). Global cycles of relative sea level oscillation are thought to be controlled by tectonic plate dynamics (Sloss, 1 979; Vail et al, 1 977). In this context, links between plate tectonics, global relative sea level oscillations, generalized subaerial exposure, plant evolution and the cycle of COz deserve much more attention than can be given here. However, it is also im portant to remember that minor subaerial exposure surfaces will be produced independently of global episodes in parts of specific sediment
packages subject to shallowing and shoaling upward sedimentation (sand shoals, reefs, deltas , tidal flats) . These local subaerial exposures may or may not be sites for development of economically significant mineral deposits but, either way, they will still be of interest for deciphering regional geology . The effects of Pleistocene sea level fluctuations in the Caribbean are recorded by relic yet distinctive mor phological features in the present day landscape. Emergent shorelines, ter races and reef tracts are common around the perimeter of many Carib bean islands . On Barbados, where gradual but continued tectonic uplift has combined with the effects of glacio-eustatic sea level changes, a se quential record of emergent coral reef terraces has been produced. On the coastal plains of Belize and parts of Florida, dolinas and cenotes pepper the landscape and testify to a Pleistocene or earlier period of karsting . In the reef tract of Belize, drowned cenotes (blue holes) have been found with their floors 120 m (400 ft) below present sea level . Such observations record evidence of karsting during a glacial, low sea level stand followed by a rise during an in terglacial . Karsting during interglacial stages of the Pleistocene has
significantly increased macroporosity of exposed carbonate terrains in the Caribbean . Similar effects, with economic benefit , are seen further back in the geologic record, for ex ample the Cretaceous El Abra of Mexico. RECOGNITION OF SUBAERIAL EXPOSURE General Procedure
The identification of subaerial ex posure surfaces in the geologic record should be based on the following con siderations: ( l ) understanding of the regional geology, including stratig raphy, structure and tectonic evolu tion, sea level oscillations, location of the regional and local paleotopo graphic highs, depositional facies and paleoenvironments , (2) detailed study of rock sequences, especially below marked discontinuity surfaces and within or at the base of continental deposits (e.g., alluvial fan , lake, swamp, glacial till, plant tufa); our aim will be eventual identification of a sequence assignable to a karst or a soil profile (Fig . 1 1 ), and (3) study o f rock types, both i n the field and in the laboratory , to identify character istic macro- and microfeatures (Table 2). Any conclusion should be sup ported with the interpretation of
22
Subaerial Exposure
Figure 34-Karst breccia. Clasts of brecciated and corroded white limestone embedded in red terra-rossa sedi ment. Note la m i n ar rind on clasts. Tubular voids are root molds. P l e i stocene. Florida.
Figure 35-Moon-milk. Located in an open joi nt of the upper vadose zone of
karst profile. Random needle fibers of low magnesian calcite {right) are pro gressively neomorphosed into m i crosparite mottles. Some of these mottles also contain r e l i cs ol corroded carbonate host, Garraf Mountains. Barcelona, Spain. Photograph courtesy of L. Pomar. Thin sect ion, XN. Scal e bar = 0.1 mm.
lithologic sequences by integrating diagnostic and non-diagnostic but commonly present features of subaerial exposure facies (as in the examples of Figs . 1 1, 1 8, 48, 52). Of these three procedure stages , we will concentrate on the last, namely the study of the rock types, but without denying equal importance of the other two. Different levels of security in the
recognition of evidence of subaerial exposure can be established from relatively diagnostic features to com monly present but not indicative ones. Features listed below, taken in isolation, occur in distinctive but completely unrelated sedimentary and diagenetic environments. But, as stated so succinctly by Bathurst (1975, p . 417), " . . . safety is in numbers and a satisfactory decision
can be reached if several criteria are combined ." At the same time, it should always be remembered that many subaerial exposure surfaces may not contain characteristic features; but absence of these features does not necessarily mean that the deposit is not a subaerial exposure facies.
Subaerial Exposure
23
Figure 36-Surficial karstification. Ir regular, V-shaped lapies filled with red brown soil and clasts of host wall rock. Note modern root penetration. Karstifica tion of Miocene limestone under a modern soil cover. lbiza, Spain.
Figure 37-Solution hole breccia. Depression in bioturbated marine Pleistocene limestone contains laminated terra-rossa, mic ri t ic carbonate crusts, with clasts of host rock toward the top. Pleistocene, Florida. Photograph courtesy of N. P. James.
Diagnostic Features
subaerial exposure.
Diagnostic indicators of subaerial exposure include recognition of karst, soils and duricrusts (Goudie, 1 973). Here we will examine only karst and caliche features, and refer only to those features of potential preserva tion in the geologic record. Positive identification of the following features, provided they are in-place, is considered highly indicative of
Indicative of Subaerial Karst Facies - The following features are
diagnostic of karst development away from the influence of supratidal pro cesses (sea water capillary fringe and spray zone). The number of diagnostic landforms is increased when considering also coastal karst . Surface landforms - Of the wide variety of surface landforms (see
Sweeting, 1 973, for example) few are of diagnostic value for subaerial (meteoric water) karst facies in the fossil record. Most useful to geologists are rundkarren forms in general, that is the rounded, smooth channels, troughs, pinnacles, 1 0 em to about 1 m (0. 33 to 3 . 3 ft) in dimension, formed under a humus cover (Figs. 1 5 , 1 6) . In exceptionally good outcrops, dolinas could also be
24
Subaerial El<posure
Figure 38-Transition zone. Calichified cracks (white} a�sociated wi th terra rossa and root penetration (R) in Miocene carbonates. Tarragona, Spain.
recognized . Dolinas (Figs. 1 2, 1 4) are circular to irregular, enclosed hollows of bowl, funnel or cylindrical shapes, 2 to 100 m (6.6 to 330 ft) deep and 1 ,000 m (3,28 1 ft) in diameter, form ed by preferential dissolution along vertical joints or by roof collapse of underlying caves. In general, any landform will be more diagnostic of subaerial exposure if covered by relic soil. Karst collapse breccia - Surface and subsurface accumulations of discrete bodies of carbonate breccia formed by foundering of cave roofs. Essential to the diagnostic value of karst breccia is the identification of collapse resulting from removal (dissolution or hydraulic erosion) of underlying carbonate rather than tectonism or sediment gravity flows. Common features to these breccias are the mixing of various types of cave sediments (sand, silt, clay), speleothem fragments and soils (Figs. 29, 34), and several generations of inplace speleothem coatings. Beds overlying the breccia may show upward attenuating V-shaped deformation (Fig . 19). The presence of organic borings of coastal organisms reduces the diagnostic value of col lapse breccia since bioerosion and wave action at the base of coastal cliffs also produce collapse breccias.
Figure 39_Caliche profile. Well developed nodular horizon overlain by platy caliche and a thin surface hardpan a t the top. This profile is developed in reddish Ple istocene loessic silts, exposed a t t he base of the profile. Tarragona, Spain.
Cavern porosity - According to Choquette and Pray (1970), "a pore system characterized by large open ings, or caverns. Although much cavern porosity is of solution origin,
Figure 40-Caliche glaebules. Nodular caliche horizon in the same locality as Figure 39. Glaebules (nodules} are soft and chalky in consistency, show tenden cy toward vertical elongation, and grew inside a red silty soil by the interaction of vertical water movement and possible
roots.
the term is descriptive and not genetic . . . . . A practical lower size limit of 'cavern' for outcrop studies is about the smallest opening an adult person can enter. Where the rock unit
Subaerial Exposure
25
Figure 41-Caliche ha rdpa n breccia. A well developed hardpan {lower half) at the top of a Pleistocene caliche profile was destroyed as part of the weathering processes formed a new soil cover (up per half). Carlsbad. New Mexico.
Figure 42-Fossilized caliche profile. White hardpan at the top of a Pleistocene caliche profile shows a gen tle undulatory morphology overlain by re· cent soil. Tarragona, Spain.
Figure 43-Caliche p rofi le Basal nodular horizon with white. chalky, ver tically elongate glaebules (nodules) separated by red sill, grC:tding upward in· to smaller. subspherical glaebules of lower part of platy caliche horizon. Tl1in (3 to 4 em) hardpan crust on upper sur· face. Pleistocene caliche, Tarragon a Spain. .
.
is known only from drilling, a prac tical lower size limit is that large enough to cause an easily recognizable drop of the drilling bit (a half meter or so) . " Large· cavern porosity can be concentrated in a subhorizontal plane, but commonly will be distributed in several intercon nected horizons (variations of the phreatic water table) . One of the reasons for the artificial lower limit of the term cavern is the possibility that smaller pores ("vugs") could form in other diagenetic environments without substantial proximity to subaerial exposure surfaces. Speleothems - Although not abun dant, speleothems (stalactites, stalagmites, flowstone, cave pearls) do occur in the fossil record (Figs. 20, 24, 28, 29), but are usually deeply
corroded and partly replaced by argillaceous micrite. These alterations are thought to result from repeated cycles of exposure in the upper vadose zone of the karst profile. In addition to in-place speleothems, reworked speleothem fragments within collapse breccias are also in dicators of karst whereas clasts of speleothems lying on a discontinuity surface are valid indicators only of proximity to karst facies. Indicative of Caliche Facies
Rhizoliths - Organosedimentary structures produced in roots by ac cumulation and/or cementation around, cementation within, or replacement of, higher plant roots by mineral matter (Klappa, 1 980a). Five basic types of rhizoliths exist: (1) root
molds and/ or borings which are simply cylindrical pores left after root decay (Figs. 56, 62, 72, 90); (2) root casts which are sediment- or cement filled root molds (Figs . 57, 62); (3) root tubules which are cemented cylinders around root molds (Figs. 53, 59); (4) rhizocretions s.s. which are concretionary mineral accumula tions around living or decaying roots (Fig. 54); and (5) root petrifications which are mineral encrustations, im pregnations or replacements of organic materials whereby anatomical root features are partly or totally preserved (Figs . 88, 89) . Most com monly, rhizoliths are formed by low magnesian calcite (LMC) accumula tions of fine-grained, white- to cream colored deposits (Calvet et al, 1975). Rhizoliths are generally millimeters to
26
Subaerial Exposure
Figure 44-lncipient platy caliche horizon. Horizontal to subhorizontal root mats penetrate and alter host calcarenite of Miocene age. Brecciated hardpan in upper part of the profile. Tarragona, Spain .
Figure 45-Horizontal root-mat with inci pient glaebular and subsurface laminar hardpan development. Millimeter-sized glaebules (caliche ooliths) resemble marine ooliths. Recent caliche profile. Granada, Spain.
centimeters (fractions of an inch to inches) in diameter and centimeters to meters (inches to feet) long. They may be abundant or absent , but when present, they are reliable indicators of subaerial exposure since evidence of marine plant roots, such as those of Thalassia grasses, is rarely if ever preserved. Diagnostic reliability of rhizoliths is further increased by their location just below breaks in the
sedimentary sequence (Fig . 55). However, rhizoliths may be confused with animal burrows; great care should be taken to separate these two different organosedimentary products (Klappa, 1 980a). Apart from rhizo liths themselves, roots are primarily responsible for, or contribute to, the formation of platy caliche horizons, some brecciation textures (Klappa, l 980b), channel and root-moldic
porosity, and alveolar texture. Roots, together with symbiotic fungi , are also responsible for the hitherto enigmatic structure of Microcodium (Klappa, 1 978). Alveolar texture (term introduced by Esteban, 1 974) - Cylindrical to irregular pores, which may or may not be filled with calcite cement, separated by a network of anastomos ing micrite walls (Figs . 65, 66). Pore
Subaerial Exposure
27
r
Figure 46-T ansi t io n and nodular zone
of caliche profile. Altered host rock (Paleocene carbonates) still preserves traces of bedding in a transition zone ot a Pleistocene caliche with incipient glaebule development. This profile is truncated by present day soil cover. Tarragona, Spain.
Figure 47-Fossil caliche. Caliche pro· file formed by a nodular horizon coalesc ing into an incipient platy caliche horizon. This profile developed on fluvial overbank silts and lutites of a Lower Triassic (Buntsandstein) alluvial fan com plex. Barcelona. Spain.
diameters commonly range from 100 to 500 I'm, but a few may reach 1 .5 mm (0.06 in). In recent caliche, micritic walls are composed of band ed LMC fibers, 0.5 to 2.0 �tm wide and from 3 to 1 20 I'm in length (Fig . 67). These banded needle fiber walls are commonly encrusted by equant microcrystalline or cryptocrystalline calcite in fossil caliche (Fig. 66). Alveolar texture may be abundant to
absent but when present is found mostly within platy and hardpan horizons. Without specifying a name, Steinen ( 1974) described and il lustrated alveolar texture and sug gested that it may represent discrete channelways within sediment which had been penetrated by rootlets. Har rison (1 977) described the same tex ture as root moldic porosity. In essence, alveolar texture is the pro-
duct of coalesced millimeter-sized rhizoliths. Microcodium Gluck - Elongate, petal-shaped calcite prisms or ellip soids, 1 mm (0.04 in) or less in long dimension and grouped in spherical , sheet or bell-like clusters (Esteban, 1974). Microcodium, long thought to be algal , inorganic, bacterial or ac tinomycete in origin, is now believed (Klappa, 1 978) to be the result of
28
Subaerial Exposure
Figure 48-Relic traces of fossil caliche below erosion surface. Sharp surface truncating the layering of a fenestral dolostone (lower part) is overlain by skeletal packstones. Thin section of the i ron-stained fenestral rock just below the erosion surface shows alveolar texture, calcified filaments, root molds, glaebules and corroded quartz grains. Permian. Guadalupe Mountains, New Mexico.
Figure 49-Calichified planar subaerial exposure surface. Near vertically bed· ded Kimmeridgian limestones and dolostones overlain unconformably by in terstratified colluvial calcareous silts and eolianites of Pleistocene age. Eolianites are calichified. lbiza, Spain.
calcification of mycorrhizae, sym biotic associations between soil fungi and cortical cells of higher plant roots (Figs . 69, 70). Tangential LMC needle fibers (term introduced by James, 1 972)-The acicular or needle-shaped calcite crystals are arranged in a band with long axes of needles parallel and tangential to the band (modified from Bal, 1 975). Sizes and shapes o f these
needle crystals are the same as those forming alveolar textures, and, in many instances, tangential LMC nee dle fibers form alveolar textures (Fig. 67). Calcified cocoons - Ovoid to spherical, 1 .0 to 3 .0 em (0.4 to 1 .2 in) diameter cases of calcified puparia of soil-dwelling insects (Fig. 64) . Record ed by Read ( 1 974) and Ward (1975) in Australia and Yucatan respectively,
these organosedimentary structures are also conspicuous in some Mediter ranean paleosols. Although not abun dant, calcified cocoons are reliable in dicators of soils and, therefore, of subaerial exposure. Caliche glaebules (including pisoliths and nodules)-A glaebule is a three-dimensional unit within the matrix of soil material, usually equant , prolate to irregular in shape ,
Subaerial Exposure
29
Figure 50-Brecciated caliche hardpan. Upper surface of Figure 49. Extensive horizontal root networks of garriga vegetation are causing brecciation of Pleistocene hardpan and leading to soil development. lbiza, Spain.
Figure 51- Transitional horizon. Relic in place "boulders" of Kimmeridgian dolostone host rock wrapped in fine grained micritic caliche matrix of lower most Pl�istocene caliche profile of Figure 49. lbiza, Spain.
and distinguished from the enclosing matrix by a greater concentration of some constituent and/or difference in fabric (modified from Brewer, 1 964). With respect to caliche profiles, glaebules consist of discrete, powdery to indurated concentrations of LMC micrite. Caliche glaebules are silt to pebble in size and may occur either as isolated particles (Fig. 73) or coalesc ed masses (Fig. 40). Many sand to
granule size glaebules show poorly developed concentrical lamination (Fig. 8 1 ) . The term caliche glaebule embraces various terms used to describe undifferentiated to concen tric structures commonly recorded in caliche facies including caliche ooids, pseudooids, ooliths, peloids, pellets, pelletoids, coated particles, nodules, concentric structures and concretions. Clay cutans -Brewer ( 1 964) defin-
ed a cutan as "a modification of the texture, structure or fabric at natural surfaces in soil materials due to the concentration of soil constituents or in-place modifications of the plasma (relatively unstable soil matrix) . " Brewer recognized several groups of cutans, distinguishable by the characteristics of the surfaces af fected, mineralogy of the cutanic material and internal fabric of the
30
Subaerial Exposure
Fig u re 52-Seq uence indicative of subo.er iHI exfJOS u re . Underlying M iocene
carbonates (M) show a flat upper sur face without positive evidence of subae rial exposure. Overlying deposits contain diagnostic evidence of subaerial exposure such as pebbles of M iocen e rock with l ichen borings (L), reddish soi ls with vert ical rhizol iths (V), and eol ianite with rhizoliths a long cross bedding (E) M a l lorca, Spa i n .
Fig u r e 5 3-Rhi zoliths . Vertical l y oriented taproot r h izoliths and subhorizontal second order rhizo l iths of Figure 52.
cutans. Cutans as defined by Brewer are pedological features and, thus, products of soil forming processes. While many pedological features such as glaebules, fecal pellets and animal burrows are not uncommon in sedimentary rocks, Teruggi and An dreis ( 1 971) believed that cutans were exclusive to soils. However, we feel that calcite cements in carbonate rocks conform to Brewer's definition
of cutans and, thus, cutans in general are not exclusive to soils. What we do believe is that some cutans are diagnostic of soils, specifically clay cutans. Although relatively uncom mon in caliche, clay cutans probably are reliable indicators of soil forma tion (Fig. 79). Circum-granular cracking (Swine ford et al, 1 958; Ward, 1 975) - Ir regular to globular masses separated
by non-tectonic fractures and produc ed by alternate shrinkage and expan sion are called circum-granular crack ing (Figs. 82, 83). A common feature in soils in general and caliche in par ticular, circum-granular cracking could also occur in intertidal sediments but , as yet, has not been clearly recorded.
Subaerial Exposure
31
Figure 54-Rhizol i t h . HoriLontal Bec.:tion of a vertical rhizolith . Area formerly oc cupied by the root corresponds to the brown central part of the rhizolith, and still there are left few remains of root moldic porosity Photograph courtesy of F. Calvet, Pleistocene eol i an i te Mallorca. Spa i n . ,
Figure 55-Caliche profile. Laminated caliche hardpan developed in a rhizolith rich horizon in Pleistocene eolianite (white sediments in lower part). Note small sink-hole indicating later karst event. Yucatan , Mexico.
Common to both
Karst and Caliche Facies
The following features, diagnostic of subaerial exposure, are present in caliche and in the upper vadose zone of karst profi1es. Lichen structures - Subaerially ex posed carbonates are commonly covered by lichens. Lichens cause tex tural and fabric changes of the upper most (up to 2 em; 0.8 in) colonized
substrate (Fig. 3 1), commonly pro ducing spongy, microsparitic or micritic layers containing organic-rich and organic-poor millimeter laminae, fungal borings, algal filaments and spheres, fungal hyphae and oily cells of fungi, reproductive structures and silt-sized to sand-sized calcite grains (Fig. 32) arranged in surface parallel layers (Pomar et a!, 1 975; Klappa, 1 979). Endolithic lichens are more ac-
tive in semi-arid climates whereas epilithic lichens dominate temperate to arctic climates. Lichen structures fossilize as borings into the rock or as calcified organosedimentary struc tures. When lichen structures show a well developed lamination they are referred to as lichen stromatolites (Klappa, 1 979). Lichen structures, in cluding lichen stromatolites are com mon on exposed upper surfaces of
32
Subaerial Exposure
Figure 56-Root molds . The top of a t h i c k bed of a s h allow marine M i ocene l imestone was colon ized by vertical roots. Rel ics of alveolar texture see n in
th in section su pport the interpretation as root molds. Mallorca, Spa i n .
Figure 57-Rhizoliths. Reddish fluvial silts s h ow vertical tracks of g rayish silts considered as rhizol i t hs. Paleocene, Barcelon a , Spai n .
both karst and caliche facies (Figs. 30, 7 1 ). We consider the presence of an extensive endolithic lichen cover (a protosoil) as the first evidence of subaerial exposure away from supratidal influence. Random needle fiber low magne sian calcite (LMC) - This term, in troduced by James ( 1 972), is a synonym of "lublinite" and "pseudomycelia" in soil science. To
produce a random needle fiber fabric , the basic distribution of acicular or needle-shaped LMC crystals is arrang ed in a random , loosely woven pat tern within voids (modified from Bal, 1 975). Ramifying, self-supporting but loosely woven LMC needles are com mon in caliche profiles and upper parts of karst profiles (Figs. 35, 84). In caliche, this fabric is common in voids within hardpan and platy
horizons. Random needle fibers also form soft , powdery nodules within the chalky and nodular horizons. Size and morphology of these needles are similar to those which form alveolar texture and banded needle fiber fabrics. In karst facies, needle fiber fabrics occur along near-surface joints and are major contributors to moon-milk deposits of the upper vadose zone (Fig . 35).
Subaerial Exposure
33
Figu re 58-Laminar caliche hardpan . Laminar micrite hardpan o f ?Pleistocene age capping reef core facies of Pliocene reef complex Everglades. Florida.
Figure 59-Rhizoliths. Planar view show ing mill imete r -sized horizontal rhizoliths occu rring just below the laminar hard· pan shown in_ Fiqure 58.
Commonly Present in Subaerial Karst Facies but Non-diagnostic
The following morphologies are common in subaerial karst but also well developed in supratidal coastal settings. Many authors refer to "marine" karst as a specific type of karst formed under the influence of both sea water (salt spray and splash zones, sea water capillary fringe) and meteoric waters . Features developed
in carbonate hosts in marine intertidal and subtidal settings are not con sidered as karst. Nevertheless, in special conditions (confined aquifers) , subterranean karst can be active below sea level . In our discussion, these special conditions are not rele vant ; we will refer only to the distinc tion between supratidal coastal karst and subaerial karst . Because of the common juxtaposition and superposi-
tion of subaerial karst and coastal karst, the separation of these karst types may seem superfluous . How ever , this distinction is necessary to be consistent with our concept of ter restrial soil and subaerial exposure. For example, extensive areas (tens of kilometers; tens of miles) of barely emergent carbonate sand shoals in the supratidal zone could produce thick sedimentary sequences without con-
34
Subaerial Exposure
Figure 60-Caliche rhizobrecc ia. Brec ciated dolomite clasts of J u rassic dolostone floatin g in a ca liche matrix of Ple i stocene age. I nset shows thin sec tion photomicrograph of xylem vessel from a plant root which was extracted from dissolved caliche mat rix between dolostone fragments. l biza , Spai n . Scale bar = 1 00 flm .
Figure 5 1 - I n c ipient caliche develop ment. Ple istocene eolianite (P) overlain by active modern soil (5). Upper deca lcified part of soil horizon c o nta i ns abundant roots. Top of this calcareous rich so il horizon (arrow b) shows i r regular dissolution surface. Subsu rface laminar caliche hardpan a n d horizontal r oot mat (arrow) overlies unaltered Pleistoc e n e eolianite (P). Excavator is responsible for smooth vertical scrape marks (i .e . . artifacts) lbiza, Spa i n .
taining recognizable subaerial ex posure surfaces. Phytokarst (term introduced by Folk et al, 1973) - " Black-coated , jagged pinnacles marked by delicate, lacy dissection that lacks any gravita tional orientation . " This razor-sharp spongework of ragged pinnacles is formed by carbonate dissolution caus ed primarily by plants. Other authors refer to very similar features as root-
lapies (deckenkarren), specifying the basic role of root penetration and presence of circular tubular sections (3 to 20 em; 1 .2 to 8 in wide) of tor tuous branching channels. Jakucs ( 1 977) noted that these spectacular lapies can reach down 25 m (81 ft) in to the rock, producing 75o/o porosity, and notes further that, under tropical conditions, their evolution is extreme ly fast (may develop even in a single
vegetal cycle of 4 to 1 0 years) . Phytokarst (or root lapies) is very in tense in tropical karst, but is also pre sent in Mediterranean karst and in supratidal coastal environments, especially in substrates with high in tercrystalline and/or interparticle porosity. In these coastal settings salt spray weathering contributes to the formation of phytokarst forms. In all these environments, phytokarst is
Subaerial Exposure
35
Figure 62-Root corros i o n . Sol utional voids at top of calcareous soil horizon shown i n F ig u re 6 1 and a r rowed (b). Tubu l a r voids (T ) are soi l -f i l l ed root molds. Arrow
(R)
points to plant root.
Figu re 63-Recemented brecc iated
cal iche hardpan. Reworked and re c emented Pleistocene l aminar caliche hardpan fragments s u r rounded by white· cream micrite. Tubu lar voids and sur faces channels (arrows) are root bor ings. Borings truncate g rains; they a re not simply root molds. lbiza, Spa i n .
associated with kamenitzas (see below) and abundant blue-green algal borings . Root penetration (terrestrial plants , mangroves, seagrasses) and bioerosion in the low supratidal intertidal zone (Litorina, Patella, chitons, Lithoph aga) are probably contributing to rock disintegration and development of both phytokarst and kamenitzas. Kamenitzas (solution pans, rock pools) - Small, flat-bottom depres sions (up to 3 m or 1 0 ft in diameter and up to I m or 3.3 ft deep) in level calcareous land surfaces (Figs . 2 1 ,
23). Sides of these depressions can be steep, vertical or overhanging . These depressions start by vertical dissolu tion of water collecting between slight undulations (modified from Sweeting, 1973). Sides are typically infested by endolithic and chasmolithic algae. Common on subaerial karst surfaces, kamenitzas also occur in supratidal environments. Rillenkarren and rinnenkarren (solution runnels)-Razor-sharp, finely chiselled, solution runnels from 1 to 2 em (0.4 to 0.8 in) wide and deep , 20 em (8 in) long, up to 50 em
(20 in) wide and deep and 20 m (66 ft) or more long (based on Sweeting, 1973), forming downward oriented (gravitational) patterns along bare rock slopes (Fig. 23) . These lapies subtypes rapidly (a few months) model other exhumed lapies (Fig. 22) and may evolve into pinnacle mor phologies (spitzkarren; Fig . 1 8). Dimensions of runnels appear to be proportional to annual rainfall . These features develop preferentially on well cemented, hard and compact car bonates . Sharp dissolution runnels may also develop on carbonates ex-
36
Subaerial Exposure
Fi g u r e 64-Calcified cocoons. Poo rly developed caliche wit h i n H o l ocene wind· deposited l ittoral carbonate sands con· tains abu ndant spherical ca rbonate structu res. Fractu red structures show smooth inner and rough outer su rfaces. Note wa lls of i nt a c t structures have holes which lead into internal chambers; interpreted as c a l c i fied cocoons (puparia) of soil-dwe l l i ng i nsects. Arrow poi nts to terrestrial gastropod. lbiza, Spai n .
Fig u re 65-Alevolar texture. Complex network of mi crite wa l l s , partially ii lled
by blocky calcite cements, between
ca l i che gl aebules. Pleistocene caliche, Tarragon a , Spa i n . Scale bar = 0 .5 m m .
posed in the coastal supratidal zone. Nevertheless, it seems that well developed rillenkarren and rinnenkar ren should be strongly suggestive of subaerial exposure. Commonly Present in Caliche Facies but Non-diagnostic
Black pebbles - Black limestone pebbles are a noticeable component in some caliche hardpans (Fig. 72). Ward and others ( 1 970), after ex amining blackened eolianite and
caliche rubble, attributed the blacken ing to the preservation of trapped organic matter within calcite crystals. They suggested that layers of fragmented dark-colored limestone may indicate subaerial exposure adja cent to hypersaline water. Klappa (unpub. data) has found that metallic oxides precipitated on fungal hyphae also produce blackening of limestone. In addition, black limestone may also form in the deep sea and in centers of large lakes where it may be tens to
hundreds of kilometers from land . Once reworked, black pebbles derived from these limestones will be difficult to tell apart from those described by Ward and others ( 1 970); study of lithofacies associations and fabric characteristics may help resolve this difficulty. Floating texture and corroded car bonate grains - Embedded, silt-sized and sand-sized grains showing no point contacts in a micrite matrix give an arrangement that is commonly
Subaerial Exposure
37
Fig ure 66-Aiveol ar text ure . A close view of alveolar texture in rhizoliths of Figure 5 4 . Pleistocene, Ma llorca, Spa i n . Photograph cou rtesy of F . Ca lve! . Thin section, plane polarized l i g ht . Scale bar = 0.1 mm.
Figure 67-Tangential l ow magnes i a n needle fibers. Par a l l e l alignment of many needle fibers form intercon·
calcite
nected b ridge -l i ke bands across voids. Pleistocene caliche, Ba rbados. Photog raph courtAsy of N. P. James. Th i n section , XN . Scale bar = 50 �-tm .
referred to as floating texture (Brown, 1 956). Floating texture has been explained in one of five ways: ( 1 ) normal sedimentary processes , {2) partial replacement of some grains by calcite, (3) partial or total replace ment of some grains by calcite with other more resistant grains of dif ferent mineralogy being unaffected , (4) expansion as a result of penetra tion and dislocation of organisms by
weathering and disaggregation of bedrock or sediment, and (5) expan sion as a result of displacive crystallization of calcite . Corrosion of non-carbonate grains and partial replacement of grains by calcite (Fig . 86) has been widely documented in caliche and is prob ably the most important mechanism for producing floating texture in caliche; however, displacive
crystallization of calcite is favored by some workers (Assereto and Kendall, 1 977; Watts, 1 977, 1 978) but this phenomenon has yet to be proved. Floating textures, although proposed by earlier workers as a criterion for recognition of caliche, can be produc ed in many environments by a wide variety of mechanisms; it is character istic but not diagnostic of caliche facies.
38
Subaerial Exposure
Figure 68-Transition zone. Intensely altered host rock (Cretaceous limestone) preserves some evidence of original bedd ing (inclined to the left) Thin caliche t1ardpan cr ust ( P l e i s to ce ne ) at the top is being destroyed under present soi l cover. The conglomeratic ap pearance of the tran sition zone (developed d u r ing Paleocene) does not imply significant transport, but is produc ed largely by root penetration ; dense Microcodium col on ies form most of t h e matrix between the Cretaceous blocks . Barcelona, Spain.
Figure 69-Microcodium. El ong ate calcite prisms with hexagonal basal sec tion forming globular agg regates in the matrix of the "conglomerate" o f Figure 68 . Thin section. plane polarized l ight. Scale bar = 0. 5 mm.
Vesicles , vesicular texture and vesicular porosity - Vesicles are spherical pores formed by rearrange ment of sediment or crystallization of solutions around fluid bubbles in an unconsolidated material . Vesicles are larger than grain-supported voids and differ from both vug and fenestral porosity principally in their walls which are smooth, simple curves with circular outlines (modified from
Brewer, 1 964) . In thin section, vesicular texture and alveolar texture are not readily differentiated, except that the latter may show tubular longitudinal sections whereas vesicular textures have only circular outlines . Vesicles are common in soils and beach sands. Vermicular texture - Dense net works of micritic tubes and rods set in a micritic matrix give vermicular or
"spaghetti-like" textures when viewed in thin section (Fig . 85). Vermicular texture forms by calcification of organic filaments (algae, fungi, ac tinomycetes). These calcified filaments are known from marine and meteoric settings and , thus, although common in caliche are non-diagnostic features. Tepee or pseudo-anticline struc tures - Caliche pseudo-anticlines are
Subaerial Exposure
39
Fig u r e 10-Microcodium. Vertical joint in J u rassic dolostone was en larged by corrosion activity of Microcodium i n tubular colonies. Black material i n open ed joint is dense. opaque a rg i l laceus terra-rossa. This joint is located in the upper vadose zone of a karst profile developed du ring Lower Tertiary. Ba rc e lon a , Spain. Thin section. plane polarized light. Scale bar = 0.5 mm. Figure 7 1 -Caliche hardpan. Multi· generation hardpan of Pleistocene age showing g l aebules (pisoliths), recemented hardpan clasts, exotic Mesozoic dolostone clast, and upper most su rface laminar micritic crust. Note inter-laminar voids within red-brown laminar crust and black rim to top layer. Laminae formed by calcification of lichen su rfaces (lichen stromatol ite). lbiza, Spain.
Figure 72-Recemented brecciated cal iche hardpan. Pink ca liche matrix with black pebble limestone clasts. I r regular brown stained anastomosing channels and tubular voids are recent root borings. Root borings through black pebbles shown i n upper left are filled with white-cream micrite. Pleistocene, Tarragona, Spain.
common in some caliche facies, but tepees or pseudo-anticlines in general can occur in marine and coastal en vironments (see Assereto and Kendall, 1 977, for a review on tepees). Commonly Present in Karst and Caliche Facies but Non-diagnostic
Irregular to smooth sharp upper surfaces, commonly truncating underlying sedimentary structures , may demarcate upper bounding sur-
faces of karst and caliche facies (Figs . 1 8, 42, 48, 49) . Shallowing upward sequences culminating in non-specified exposure surfaces, missing paleontological zones, or in-place non-tectonic frac turing and brecciation are also com monly present . Other features common to non diagnostic karst and caliche facies in clude: microscopic features such as clotted micrite (structure grumeleuse;
Figs. 80, 91), microspar, meniscus cements (Fig. 77), gravitational cements (Figs. 77, 78), calcified filaments (Fig. 85) , calcified fecal pellets (Fig. 88) and microborings (Fig. 76); leached and vuggy porosity; lithoclasts in beds directly above sedimentary breaks (Fig. 50); laminar micritic crusts; chalky, friable to powdery white deposits (more diagnostic of subaerial exposure when below sedimentary break); iron oxide
40
Subaerial Exposure
�tgU!f.' 73 -�l<��bu �:�s (piool:ths}. c etaceo.Js limast()'le (6owP.r :::a ) ,:; sllarpty eroded .a r:d ov�r!:.,;tl tw � pi:>cl•th grai:"l!lOO:"le (Pa l�oc�r 11':;!) G1aeb:..J ta s (piaolilhs) t:�iginat9:1 ;n hypothetic�! Mat by calicl"le proli le b1r: wer� lrd1 : spo ;ed later5.1�y ar;d resedirnent�d or. st:arp aro· s! Oil SL'rfa� . Sed iMI:!:'1t.;� ry tr:m:lpwt �mj oo� br. ;�erl:l.t�d �Hain �ic:e oc rtl n� in ca��sening \J pweJd aeqt: ences ·J ar· re.gcm�, Spair: .
staining (Figs. 48, 73) ; and crystal silt (Dunham, 1 979a) which consists of geopetal fillings in secondary porosi ty. Crystal silt can be generated from the chalky horizon of caliche profiles or from moon-milk deposits of the upper vadose zone of karst profiles . This fabric is present in subaerially exposed carbonates but may also be produced in submarine environments (e.g. chips from boring organisms). Features Related to Meteoric Diagenesis
Since the late 1 950s, fresh water diagenesis has been considered of ut most importance by carbonate geologists, but perhaps has been overemphasized or misunderstood. It is pertinent here to remember that vadose diagenesis, leaching and moldic porosity are not necessarily related genetically and do not imply the development of subaerial ex posure surfaces (although they can occur close together). Negative
Figure 74-Caliche hardpan. I rregular,
subhorizontal crust lami nae are
underlain by brecciated Pleistocene limestone and separated by darker car bonate with abundant white g laebules. Barbados. Polished slab, scale bar = 2 em. ( From James, 1 972; his Fig. 4a ) .
isotope ratios are widely accepted as evidence of meteoric water influence. But many criteria considered as diagnostic in the late 1 960s are now known to occur in very different en vironments. For instance, although pervasive calcite cementation , calcitization of dolomite ("dedolomitization") and aragonite, leaching of carbonate, syndeposi tional brecciation, growth of poikilotopic calcite, development of vuggy porosity, production of 'crystal silt' and dolomitization by mixing waters do occur under the influence of meteoric fresh water, many of these processes are also known in submarine environments (Lindstrom, 1 963 ; Shinn, 1 969; Schlager and James, 1 978). Besides, the emphasis of our discussion has been the charac terization of surfaces of subaerial ex posure and their recognition in the geologic record , the processes listed above are not necessarily related to a surface of subaerial exposure. Caution should be exercised in the use of these criteria as evidence of
subaerial exposure. Recent studies show increasing uncertainty in the ab solute distinction between meteoric and marine diagenesis (Schlager and James, 1 978; Klappa and James, 1 979). Phreatic diagenesis, in par ticular , is an obscure and poorly known world. CLARIFICATIONS Palustrine Carbonates
One confusion zone in carbonates is the marginal areas of lakes. There is a continuous spectrum between lacustrine carbonates, palustrine (swamp, marsh) carbonates and caliche. Freytet ( 1 973) pointed out marked similarities between fabrics of palustrine carbonates and fabric� of caliche. Palustrine environments are areas where lacustrine carbonates (or relic subaerial soils) are subjected to incipient soil processes (colonization by land plants, formation of glaebules, circum-granular cracking) but still saturated with lake water (for this reason they are also termed hydromorphic soils). We recognize the problem of distinction between palustrine carbonates and caliche but this problem can perhaps be resolved by studying vertical profiles and their lateral facies relationships, sup-
Subaerial Exposure
41
Figure 75-Laminar caliche hardpan. Subhorizontal micritic to mic rosparitic carbonate laminae; laminae i n center contains abundant microglaebules. White specks are silt·sized quartz grains. Quaternary caliche hardpan, Barbados. Photograph cou rtesy of N. P. James. Thin section, plane polarized light. Scale bar = 0. 5 m m .
Figure 76-Caliche hardpan. Clotted micrite overlain by laminar micrite. Spar· filled voids are root molds. Metallic oxide encrusted filamentous microorganisms in vertically oriented root mold. Poo rly developed alveolar texture just left of center. Pleistocene. l biza, Spain. Thin section, plane polarized l ight. Scale ba r = 1 00 pm.
plemented by geochemical analyses , rather than studying fabrics alone. Superficial Similarities
Laminated carbonate crusts -
Algal stromatolites, lichen stromatolites (Fig. 71), " case hardened" carbonate crusts, flowstones, laminar tufas (Figs. 92, 93), surface and subsurface laminar caliche hardpans (Figs . 45 , 61) all show some similarities. However, dif ferences do exist. These differences may represent formation in very dif ferent environments and can be recognized, usually in thin section and , with fortune or experience, in hand specimen. Many references exist to laminated micritic crusts, in particular to the laminated crusts of Florida (Multer and Hoffmeister , 1968) as support for the soil origin of some rocks. These Floridian crusts show a non laminated microcrystalline rind coated by porous and non-porous laminated micrite . The microcrystalline rind is probably a relic part of a pre-existing caliche profile. But the laminated micrite is formed by repeated calcite precipitation in layers of plant debris (plant litter) and cannot be considered as a true soil (as pointed out by Multer and Hoffmeister, 1 968). Laminated crusts of the Floridian
type can be present in caliche hard pans and on karst landforms; they are indicators of subaerial exposure . In essence, these crusts can be con sidered as micritic plant tufa, but they do not necessarily imply the presence of developed soil profiles or prolonged subaerial exposure; these laminated crusts can occur in barely emergent shoals. The intention here is not to cover
petrographic characteristics of each crust type but, rather, to advise deliberation before reaching a conclu sion on origin and significance of a particular carbonate crust . Carbonate Pisoliths - Pisoliths, concentrically laminated spheroidal bodies larger than 2 mm (0.08 in), form in many environments. They are not restricted to caliche facies but oc cur also in shallow marine, deep
42
Subaerial Exposure
� 1.re n �r�vit3tion..'ll ar:o mer)i'>CU:"O 'IS.do:;� cernent!:l. Cal ictllfi�d eoiiar11t� cf l'leiat¢cer'le age. W�l-ro-Jnded S<:�'ld· si� cart'Jenate .grmM in Aliz.ar;r.e re<:l · S sta;ne<;� calictle mknte. Spa�r, calcite f::ctwee n grain� :n ! O'NSr part $how met'l iscu� cutl ine:o . t..sym i'Y'IP.tncal gr�"':-:a· tJonal or pendulilN OCI':'lCIE ben�th gram lr- 1o•ue · lett. :blv.t, �11'1. ll !ln see· 1ion. pEane f>Oid ri71;!d I; ght. Rc:
marine, peritidal, cave and lake en vironments. In some of the literature of the 1 970s, pisoliths have been con sidered as diagnostic of subaerial ex posure, mainly those showing polygonal fitting, downward elonga tions and perched sediment inclu sions, occurring in beds with coarsening-upward graded bedding and affected by syndepositional, non tectonic fracturing. Although some caliche hardpans and speleothems can show similar features, Esteban (1976) questioned their value as evidence of either subaerial exposure or develop ment of caliche soils. Many of these features are simply the result of in place fabric evolution under some in fluence of hydrologic vadose regimes. The environment of formation of the different types of pisoliths can be learned only after detailed study. On ly then will pisoliths be of diagnostic value.
FI� J I € 78- M icw.:;t:;J�ctitic ;.adcse ce .-r.�"rt. Mlcm&talacH.c or pa.'\du!MI tibrcus caicite � f"rmcr r1;of .JF �t.artz· lich r.:Q itd:e M rdc�. l¢we:· s;o.�; of lM t:�hooomicrcg �· s?lo\'-iS mlcrogfa'3bu:ar !�l.v ic. P�eistoccnc. l biVl. Sp:!ir'l . Thin section. piMe po!arile� ' igh1, Scale ba: -
0 . 5 mm.
Figw·e 79-c4a.y cutan, Chalky horlzos1 of caliChe proifie oom;,e�soo nf �i:�.sizcn cal<;ite cr ·15�als {1nic rocspar). rormc1 w1� l l ol 'fOkl n o-.<: : in·3C: with rf3c f� rrlc·rfcl' t:(Yl · caroo M te cl<.�y �Jt-lr. {arrows). !bi a, .S pt: ir. . Ti 1i11 �t�t:t!t)r.,
Pleiertocene,
pln•e
,u m .
ooia ri 7.t11 w
I I{Jhl. �al� bi:lr
=
1 00
along the Persian Gulf shorelines . Coastal environments represent Most authors referring to coniatolites belts between submarine and ter (or "marine caliche") mention super restrial realms and show attributes of ficial similarities with caliche hard both marine and subaerial influences . pans or speleothems and also possible The coastal environment is also a partial analogies with the widely confusion zone containing products known "vadose pisolite" facies of the commonly referred to as coniatolites Permian of west Texas and New Mex ("marine caliche") and coastal ico (Dunham, 1 969b), but their marine karst. The term coastal features are not diagnostic of sub marine karst is most adequate, but we aerial exposure or soil development have already mentioned the reasons (see Esteban , 1 976, for a discussion) . for its separation from the subaerial A s a conclusion, w e believe that the karst. Here we will refer briefly to the isolation of the coastal supratidal and considerable discussion brought about intertidal environments from the con by different interpretations of con cept of subaerial exposure s.s. will iatolites and their possible fossil benefit the accuracy of paleoenviron analogs. mental interpretations. The terms coniatolite (Purser and Loreau, 1973) and "marine caliche" ECONOMIC CONSIDERATIONS (Scholle and Kinsman, 1 974) refer to aragonite and high magnesian calcite Oil and Gas Exploration encrustations with a variety of mor It is well established that many oil phologies (laminated crusts, drip fields are related to unconformities . stones, pisoliths) occurring in the in Chenoweth (1972) notes: " In view of tertidal and supratidal zone (Fig. 94) . this close association of petroleum These products are well developed with unconformities, one would Isolating the Coastal Environment
Subaerial Exposure
43
TABLE 1
Horizon
Average Thlckneu
Poroalty
(em)
Boundary 1\'Pe
0. Soil
32.5
Very abrup1
1 . Hardpan
20.6
clear to diffuse
% Field Eatlmete
Channel
to clear
-
30 -
Cementation
%
Point Cementing Counting Agent Continuity discontinuous Minor calcite
0·1 3
Strength weak
calcite :t:
continuous
indurated strong
silica
62.6
diffuse
fracture; moldlc
10
3·17
calcite
continuous
3. Nodular caliche
1 1 8.2
diffuse
interparticle
10·1 5
3·23
calcite
discontinuous
3. Nodular chalky horizon
89.0
gradual to diffuse
interparticle
1 5-20
calcite
discontinuous
weak
4. Chalky
54.5
Interparticle
15·20
9-35
calcite
discontinuous
weak
fracture; vug
1 5-25
3-31
calcite
diecontinuous
2. Platy caliche
4.
horizon
5. Transitional
horizon
gradual to
diffuse
1 42.0
strong to
weak
weak to
strong
Table 1 . Physical properties of Quaternary caliche profiles (based on 326 measured profiles from Spain).
assume that concentrated studies of unconformities would occupy much of the time and talents of explora tionists. Paradoxically, this is not so , and the unconformity. . . remains imperfectly understood ." One possi ble reason, among many, for the paucity of detailed studies of the types, causes, and effects of uncon formities is their non-diagnostic response in well-logs and seismic pro files. Without extremely refined techniques, now in development, dif ferent types of unconformities or ex posure surfaces will be recorded as non-diagnostic kicks and reflectors. Many, if not most, of these uncon formities have suffered effects of subaerial exposure. During periods of subaerial exposure, carbonate sediments or rocks are susceptible to diagenetic changes which may either create or occlude porosity. Integrated regional studies, together with recognition of features outlined above, may help in understanding processes operating during subaerial exposure and, thus, in predicting pat terns of porosity distribution. Porosity potential is usually high in a paleokarst facies. A good example occurs in the Tampico region of Mex ico. The Golden Lane trend follows
an elevated atoll-like rim of Albian Cenomanian reef limestones (El Abra). This elevated platform edge was subjected to post-Cenomanian karstification and subaerial erosion before being buried by sealing rocks (shales) of Upper Cretaceous and Paleocene age. A similar age of karstification affects the Mesozoic se quence in many Mediterranean coun tries, locally prolonged up to the Lower Miocene, producing relatively important fields in the area (for ex ample, Amposta field). A different example is the Arbuckle Limestone of the Central Kansas uplift . During both Late Ordovician and Late Penn sylvanian times, a karst landscape developed on the Arbuckle surface. Leached residuum, I to 10 m (2 to 30 ft) thick, formed a mantle during these karst episodes and acted as the Arbuckle reservoir rock (Walters, 1 946). In general, seals for karst reservoir traps rely on subsequent sedimentation above the subaerial ex posure surface. Caliche facies, on the other hand, generally have low porosity potential. Caliche facies are not by themselves targets for oil exploration because their volumes and reservoir qualities are generally below economic viabili-
ty; in addition, their sealin-g potential, even of the hardpan, can be reduced dramatically because of fracturing and brecciation (tectonic, collapse, root penetration). Nevertheless, the discovery of caliche facies provides an optimistic signpost to possible underlying or juxtaposed karst reser voirs.
Otber Mineral Resources Lead-zinc deposits Stratabound -
Mississippi Valley-type lead-zinc deposits (Brown, 1 970) are common in some thick sequences of carbonate rocks, particularly dolostones, which have been subjected to major episodes of karstification . Whether introduction of metal-bearing fluids and the cavity forming processes of karstification are related genetically or not, is a debatable point (Bernard , 1976). Some authors use the term "thermokarst" for cavity forming processes by hot waters irrespective of their origin and assume a hydrother mal origin for these metallic deposits. For other authors, the hydrothermal metal-bearing fluid is thought to precipitate in a subaerial karst facies. Finally, other authors emphasize the role of vadose meteoric diagenesis and evolution of the deep phreatic
Subaerial Exposure
44
1-.Sur�:::� (1(; -\":lo r.ed
r."11critt?
If\ .
.
( tEl C.oi::ElC
filbrit; :r. for med h:,- dP. r. !;�;:> rnic rti:! g laehr. l gs •::Je.u:ct�) �I: uWii i•J sex r: � h · i · p ierol c r:.rmbi;· ;':-actl.!ring (f) �·� •= is ..,O'iv
hard ai)C coroolete ly oe:Yiem€<.1, s � #ing
:!g;d �racturing. MicrocY.taium !ragn1en1 ( M). Pa:coccnc:. B a rcelona, Sr.a:n .,..h ln �e:chon, �ant: pol�:� !.£cu lt� t.: . Seal� bli.r 0.55 11' 1 !1. .
=
figu(e o 1 -GS (prsoli1tl·;;) �oorl!f· Jarrr r�!:lteo. cor�e�rmic m icritic � rains ir P:eistocerre cal•che har-;; pa r. Wl:ite spe-cks &1'e detMal q� .:mz -;) rn.in:. Tauagona, �in. Thin section. plane.: pclariz� l!ght. Sc�:e b::Jr U.5 r:1rn . -
(imbibition) zone as the adequate steps in the concentration process of formerly dispersed mineral fractions . But most interpretations need a physicochemical trap of metal-bearing fluids by impermeable fine-grained sediments . Therefore, karst develop ment produces both reservoir and trap for this type of mineral precipitation. Bauxite deposits Surface and -
subterranean karst landforms are well known containers of the majority of world bauxite reserves (Valeton, 1 972; Nicolas and Bildgen , 1 979). Lateritic soils and terra-rossa may evolve in place into bauxite; karstification may take place before, during and after bauxite mineral formation . Bauxites are subject to important resedimenta tion processes by fluvial systems dur ing different episodes of their
diagenetic evolution. Bauxites may also form in an allochthonous cover (eolian, fluvial) on karst from detrital components eroded from adjacent ig neous rock bodies . Bauxites are known in the Late Proterozoic and Paleozoic, but major ores occur in Mesozoic and Cenozoic subaerial ex posure facies, mainly in those related to major global sea level changes in the basal Eocene and Cretaceous.
Subaerial Exposure
45
figvre 82-C> �,ur rro;:� r�•: lJIC!l c��cking �liotlr: :.,afd.;=��n wilh r.l)n·laml.1at� !} e.ebu les. AJtered cenlr�l :;trHir: sh:rws pa!tlal ;eachin� and c7ac�J:"lg a.-ourld ttre ouii'X m'lrgm. �1ooc ar:9. �are$IC!n .9 , &o<1ir1 Thi' l f*� :ti<>n . plar.� po la ri:?M l{sht.
Sc�lc
Uranium deposits --:- One of th.e most important types of settings for uranium accumulations are lacustrine, palustrine and caliche carbonates . In recent years , caliche profiles as uranium traps have received much at tention . The mechanism of uranium accumulation implies lateral ground water transport of uranium-rich solu tions originated in extensive outcrops of granite and regolith. Precipitation to form carbonate-carnotite ore bodies occurs within the capillary fringe and the soil moisture zone . Evaporative concentrations, loss of COz, common-ion precipitation and oxidation are thought to be. control ling factors of precipitation . Water Economic considerations on subaerial exposure surfaces should not neglect that many areas of the world, especially those in semiarid and arid climates, rely on karst systems for their vital water supply. -
CONCLUDING REMARKS
Subaerial exposure surfaces in car bonate sequences are recognizable by
a break in the record and by specific diagenetic facies superimposed upon exposed carbonates underlying these surfaces. These diagenetic facies are karst and soils. According to Jakucs ( 1 977), karst is the formal imprint
upon the soluble parent rock of chemical and biological evolution of the overlying soil cover (including protosoils). When considering soils, emphasis here has been on caliche soils because of their high preserva tion potential in the geologic record . Sediment texture of both karst and caliche facies will be governed by original substrate characteristics, in tensity of subaerial diagenetic pro cesses, and duration and direction of profile development . No generaliza tions can be made except perhaps that there is an overall trend toward obliteration of precursor facies and generation of fine-grained, commonly friable, low magnesian calcite pro ducts in both karst and caliche facies. Modifications of post-Silurian subaerially exposed carbonates is con trolled largely by direct or indirect biological processes which are ex pressed by the accumulation of biogenic products in the upper vadose zone. Before the Silurian, on the other hand, subaerial exposure facies are expected to differ from those described in this paper . Karst and caliche facies , as known today, will not be represented before the advent of substantial land plant colonization toward the end of the Silurian. The interpretation of subaerial ex-
��r
=
0.5 'rH!l.
Figu1e 83-Cn c:u r� r-g ranutar Md : n .
r.r;agranu t�r c rar.f
oa:ich� m<Wiz � O'J1npJ.a.x �aue-r:"l� of ¢(Qc:k: r-Q a 101-II)C e.nr.t 'l"'lt11 "l gre.ir.s.
Crac�<; ng >WJl;a:Jiy o ngi(l3led 'oly ��ted v.'$tt:ng � r.d drying. White <3�cks are QtJ3r17. gra·r:s. Pk!istoco:m e. Tarragcr:&, Spai:'l. Thin sectic.n. plar:c pola�ized light. Scale bay 0 .5 r:1m. ,_
46
Subaerial Exposure
Figure 84-Random low magnesian calcite needle fibers. Random needle fibers of calcite form a loosely woven network within primary and secondary voids. Quaternary caliche hardpan. Bar bados. Photograph courtesy of N. P. James. Thin section, XN. Scale bar = 1 00 f.lm.
Figure 85-Calcified fi laments. Network of i ntertwined calcite tubes, each with a coat ing of m i crite, poss ibly sloug hed-off calcified root hairs. Pleistocene caliche. Barbados. Th i n section, plane polarized light. Scale bar = 1 00 f.lm. (From James, 1 972; his Fig. 6d).
;g,.He 8ti
Co�YOOe
t�>.tute . l rregLdat �r.c.J. lt:'tc s� l�·�il� �uartz grair, s lloa,in� In c.allch� mic mt.par C..1.l:che har4�ar. 01 Ptei �lOVt!ne a� eo :a, ra�oo-a , .Spo.i't Th:n �l:': IIO!I, XN 'N!:h 0�9�utn pl�lt!. Scai� bar 1 00 t: m . =
posure facies presents many dif ficulties: ( 1 ) morphologies and pro ducts of both karst and caliche facies are not necessarily in equilibrium with the subaerial environment . Relic features are very common and may represent geologically long time spans (tens of millions of years); (2) par ticularly frustrating is the fact that many subaerial exposure facies undergo erosion and/or coastal to marine exposure and any evidence of
subaerial weathering can be wiped out before being buried by new deposits; (3) duration of subaerial exposure cannot be measured by the thickness of the weathering profile or by the in tensity of diagenetic transformation; (4) the intensity of diagenetic pro cesses in the weathering zone is not necessarily related to proximity of the subaerial exposure surface. Both thickness of subaerial exposure facies and intensity of diagenetic processes
are also controlled by hydrologic pat terns, host lithology and climate; and (5) because subaerial exposure sur faces are commonly accompanied by erosion, in many ancient examples we have left only the lower p art of weathering profiles . This may give the impression of incipient or poorly developed subaerial exposure diagenesis (weathering). The main purpose of this paper has been to provide adequate criteria for
Subaerial Exposure
F:gt.r e- HI
47
Corroae:tl
caroo.·1a:e QYar(I. t:CY.ed (IJy :oot h�.:.-s) �i· iai i.j torom ir. c lctted n'li� r it� matrile :Mti1 spar f11J€1d ::: har. 11�1 vc� ds. T(e.ilsl· tioMI nc: ri;;:o"' nf P!elsl�er.e c�Jicht: uv�rlying mixed tsr rcF0»-'>2 wrd �c;: ian:�.e aiSoJ of r'! Bist·:JI::� ne ane. lbiz3, �ill.
·:..'.o�rodcd a....::
Tlrir: sedc;r., p!ai'W! ;X>Iaf.?�?d So>::lle bar � oc ,m .
l�h�.
._,
Rgure �-P.h.z.o.!�n al'lrl C!il;:dierJ le<.<:1 pel : ets. Oe ii,.J I;Jr pattem � �ble enc:ruste<J p!ant cel l wa lls o1 roor. &Jb�ohe ric.ai �arod·S:?ed mlcrl11c �... liclo;.s ;,v� �co.lcr.� fecal peiiP.� pro-.n.. c·
ac tr{ !ormN rc.:>t·f&.:�dln!-l t.\f�<J r! isrns. Plt!;stocene c.al11�t"� :-r.ar� r re� r�<�som�.
SDaln. Tt;i r! �ectton. 0.� :T: r!'l.
the recognition of subaerial exposure surfaces in the geologic record . Our emphasis has been on rock facies rather than processes operating in these facies. We have offered a set of lithologic features with different diagnostic values from highly diagnostic to commonly present but non-diagnostic (Table 2). We believe that a careful use of these features, in conjunction with larger scale
stratigraphic studies, will provide an adequate base for the identification of subaerial exposure facies. As a final comment , we would like to give perspective to this contribu tion. This study does not pretend to be exhaustive or definitive; rather, it represents the first attempted mar riage between partners hitherto thought mutually exclusive, the karst facies and the caliche facies . We have
XN .
.Sea:� Mr
=
tried to confine our discussion to geologically relevant topics but, in so doing, have neglected important classical approaches of other disciplines. Despite these shortcom ings, we hope this contribution will be a spring-board toward greater refinement.
48. Subaerial Exposure
-��ure 89- -ftn i�lill' {l<.lOl !�:m: f�� t-or. ) CJicite-fil;ec· c-ell.s ?t ���"'j ront s�."l.r fJI: Cd �Ub•Jlar •.:od :; !VJ Sh� i r. lr!H1!WC15e seclicr: -� circu lar str�.;ctufes ar-a 'Josit.ons of �he vae r.ul:�r l-:y�le-1'. r-Jc.rnJic H OOf i z-::> r. (;f Pl� is1·X:$Pt' Ci;lfC.IJ� de'leloped 1(1 ?le:swc�n� �:, lien:t.:! l bi:ta. �aln J l",h :1ectiM , pl�r.e po.atil� : iQil l . Scale I:: a • = 1 2� r.rn
ACKNOWLEDGMENTS
SELECTED REFERENCES General References on Exposure Surfaces
Ideas nurtured in this compilation were born in Catalunya and the Balearic Islands (easlern Spain) and reared in discussion and collaboration with L. Pomar and F. Calvet (both from the University of Barcelona) whose contribu tions, documented in the form of doctoral theses, sharpened our appreciation of subaerial diagenesis . During field studies, at various times and in different places, L.
Allan, J. R . , and R. K . Matthews, 1977 , Carbon and oxygen isotopes as diagenetic and stratigraphic tools; sur face and subsurface data, Barbados, West Indies: Geology, v. 5, p. 16-20. Chafetz, H. S . , 1 972, Surface diagenesis of limestone: Jour. Sed . Petrology, v. 42, p. 325-329.
while and enjoyable. Esteban acknowledges partial support from Consejo Superior de Investigaciones Cientificas (lnstitut Jaume Almera) and Memorial University of Newfoundland. Klappa acknowledges receipt of a Natural Environment Research Council Student
Goudie, A . , 1 973, Duricrusts in tropical and subtropical landscapes: Oxford, Clarendon, 1 74 p. Jaanussan, V . , 1 96 1 , Discontinuity sur faces in limestones: Geol. Inst. Univ. Uppsala Bull . , v. 40, p. 22 1 -24 1 . Klappa, C . F . , 1 979, Lichen stromato lites; criterion for subaerial exposure and a mechanism for the formation of laminar calcretes (caliche): Jour. Sed. Petrology, v. 49, p. 387-400. __ and N. P. James, 1 979, Biological ly induced diagenesis at submarine and subaerial carbonate discontinuity sur faces (abs .), in Recent advances in car
Pomar, F. Calvet, L. C. Pray, R. Salas and J. F. Meeder provided support, refinement and logic to our observations . W e acknowledge with thanks N . P . James, L. Pomar and F. Calvet for kindly lending photographs and R. G. C . Bathurst whose constant encouragement and enthusiasm made our struggles worth
shi p while at the Un iversity of Liverp o ol ( 1 975-1978) and a Memorial University of
Newfoundland Post-Doctoral Fellowship ( 1978- 1980) .
bonate sedimentology in Canada, a C . S . P . G . symposium : Calgary, Cana dian Soc . Petroleum Geols . , p. 1 6- 1 7 . Martin, R . , 1 966, Paleogeomorphology and its application to exploration for oil and gas (with examples from western Canada): AAPG Bull . , v. 50, p. 2277-23 1 1 . Multer , H . G . , and J . E. Hoffmeister,
1 968, Subaerial laminated crusts of the Florida Keys: Geol . Soc. America Bull . , v . 79, p . 1 83- 1 92. Newell, N . D . , 1 967, Paraconformities, in C. Teichert and E. L. Yochelson, eds . , Essays in paleontology and stratigraphy: Lawrence, Kansas, Univ. Kansas Press, p . 349-368. Pomar, L . , et al, 1 975, Accion de li quenes, algas y hongos en Ia telodiagenesis de las rocas carbonatadas de Ia zona litoral prelitoral Catalana: Univ. Barcelona, Inst. Inv. GeoL , v . 30, p . 83- 1 1 7 . Rose, P . R . , 1 970, Stratigraphic interpre tation of submarine versus subaerial discontinuity surfaces ; an example from the Cretaceous of Texas: GeoL Soc . America Bull . , v. 8 1 , p. 2787-2798 . Schneider, J . , 1 976, Biological and inor ganic factors in the destruction of limestone coasts: Contr. Sedimen tology, 6, 1 1 2 p . Semenuik, V . , 1 97 1 , Subaerial leaching in the limestones of the Bowan Park Group (Ordovician) of central western New South Wales : Jour. Sed. Petrology, v . 4 1 , p. 939-950. Videtich, P. E . , and R. K. Matthews, 1980, Origin of discontin uity surfaces i n limestones; isotopic and petrographic data, Pleistocene of Barbados, West In dies: Jour. Sed . Petrology, v. 50, p . 97 1 -980.
Subaerial
Exposure
49
TABLE 2
A. Dia (a)
�ostic Features
iagnostic of Karst facies 1 . Karst surface landforms 2. Karst collapse breccias 3. Cavern porosity 4. Speleothems (b) Diagnostic of caliche facies 5. Rhizoliths 6. Alveolar texture 7. Microcodium 8. Tangential LMC
needle fibers Calcified cocoons Caliche glaebules Clay cutans Circum-granular cracking (c) Diagnostic of both karst and caliche facies 1 3. Uchen structures 1 4. Random LMC needle fibers 9. 1 0. 11. 1 2.
B. Commonly Present But
Non-diagnostic Features
(d) Commonly present in karst facies 1 5. Phytokarst 1 6. Kamenitzas 1 7. Rillenkarren (e) Commonly present in caliche facies 1 a. Black pebble limestone 1 9. Floating texture and corroded grains 20. Vesicles and vesicular texture 2 1 . Vermicular texture 22. Tepees ( f ) Commonly present in karst and caliche facies 23. Irregular to planar sharp upper surfaces 24. Shallowing-upward sequences
culminating in
non-specified exposure surface
25. Missing paleontological zones 26. In-place non-tectonic fracturing and brecciation 27. Microscopic features: clotted micrite, microspar, meniscus and gravitational cements, calcified filaments, calcified fecal pellets, microborings 28. Leached and vuggy porosity 29. Lithoclasts in beds directly above sedimentary breaks 30. Laminar micritic crusts 3 1 . Chalky layers below a sedimentary break 32. Crystal silt
Table 2. Summary of features i n subaerial exposure facies.
Karst Facies Bogli, A., 1 980,
Karst hydrology and physical speleology: Berlin , Springer Verlag Pub . , 284 p. Donahue, J. D., 1 969, Genesis of oolite and pisolite grains; an energy index: Jour. Sed. Petrology, v. 39, p. 1399-141 1 . Folk, R. L., and R . Assereto, 1976, Com parative fabrics of length-slow and length-fast calcite and calcitized aragonite in a Holocene speleothem , Carlsbad Caverns, New Mexico: Jour. Sed. Petrology, v. 46, p. 486-496. __ H. H . Roberts, and C. H . Moore, 1973, Black phytokarst from Hell, Cayman Islands: Geol. Soc. America Bull . , v. 87, p. 235 1-2360. Jakucs, L . , 1 977, Morphogenetics of karst regions: New York, John Wiley and Sons, 284 p. Kendall, A. C . , and P . L . Broughton , 1 978, Origin of fabrics in speleothems composed of columnar calcite crystals:
Jour. Sed. Petrology, v. 48, p. 5 1 9-538. Legrand, H. E., and V. T. Stringfield, 1 973, Karst hydrology - a review: Jour. Hydrology, v. 20, p. 97-1 20. Purdy, E. G., 1 974, Reef configurations; cause and effect, in L. F. Laporte, ed . , Reefs i n time and space: SEPM Spec. Pub. 1 8, p . 9-76. Quinlan, J. F., 1 972, Karst-related min eral deposits and possible criteria for the recognition of paleokarsts; a review of preservable characteristics of Holocene and older karst terranes: Montreal , 24th Internat. Geol. Cong. , Sec. 6 , p. 1 56-168. Sweeting, M. M . , 1 973, Karst landforms: London, Macmillan Pub . , 362 p. Thrailkill, J., 1968, Chemical and hydro logic factors in the excavation of limestone caves: Geol. Soc. America Bull . , v. 79, p. 19-46. __, 1976, Speleothems, in M. R. Walker, ed . , Stromatolites, developments in sedimentology 20:
Amsterdam, Elsevier Sci. Pub . , p. 73-86.
Caliche Facies and Soils in General
Aristarain, L. F. , 1 970, Chemical analysis of caliche profiles from the High Plains, New Mexico: Jour . Geology, v . 7 8 , p. 201 -21 2. Bal , L., 1975, Carbonate in soil; a theore tical consideration on, and proposal for its fabric analysis . I . crystic, calcic and fibrous plasmic fabric: Netherlands Jour. Agr. Sci . , v. 23, p. 1 8-35. Blank, H. R., and E . W. Tynes, 1 965 , Formation o f caliche in situ: Geol . Soc. America Bull. , v . 76, p . 1 387- 1 392. Braithwaite, C. J . R . , 1975, Petrology of palaeosols and other terrestrial sediments on Aldabra, western Indian Ocean: Philos. Trans. Royal Soc. Lon don, v. 273, p . 1 -32. Bretz, J. H . , and L. Horberg, 1 949, Caliche in southeastern New Mexico : Jour. Geology, v. 57, p. 491-5 1 1 .
50
Subaerial Exposure
r· gure 00-Hool •rt�ds. Obliq�Jt; sec1 1on ?: 1 1 �1;5[. 1 I GCI l?l.::! ld!l 11\'i thin 'ICJW lX!I ' ll�n:�c .::haJ k�' ("n:croSPar c• sill-size� ca1txJnat�) horl:c:n o• 1-ll�t$1-::c t!r.P. c:!l: l�he . Open lt""dr.hJre 1\JI�f:illg 1-l W-SE p: IS !-d<JIB!i ::ra•:sected root tr..;'lkJ. ;�ll!a, �.lpain . Thr. Mction , pl-'!11e poi;";�riz.ed ; igh1 Sc.;�le bar 100 )llll =
:Ci{.!u�.;;: �1 -il lv:pi<:l lt � 1.\0:: t��.o�:e,:; ard r.har: MIS Mat�:): l:'l canct: P. l'!ardpa n w·itn &bund�nt � i:H;zed ft�r,�rtt.;.'ll!:i or sKeletal g.-air.s ar.d oohtal Q:.lartz, *-owl"lg lne; . c ien; Cl·frerentiat icn iflt'> :r: ic rile glaetx.Jte-s �.,.. mlcrcspar cMr.ne ls. Pleistocene, :arra�on�. Sp&: n . I hlr: S(IC · tion, J:iane poi31':1M l ighL
Scale
oar
=
0. 1
r:tm.
Brewer, R . , 1964, Fabric and mineral analysis of soils: New York, John Wiley and Sons, 470 p. Brown, C. N . , 1956, The origin of caliche on the northeastern Llano Estacado, Texas: Jour. Geology, v. 64, p. 433-457 .
Calvet , F., L. Pomar, and M. Esteban, 197 5, Las rizocreciones del Pleistoceno de Mallorca: Univ. Barcelona, Inst. In vest. Geol . , v. 30, p. 35-60.
Durand, J. H . , 1%3, Les croutes calcaires et gypseuses en Algerie; formation et age: Soc. Geol. France Bull ., v. 7, p. 959-968. Esteban, M., 1 974, Caliche textures and Microcodium: Soc. Geol. Italiana Bull. (supp.), v. 92, p. 105-125. , 1976, Vadose pisolite and caliche: AAPG Bull. , v. 60, p. 2048-2057. Gile, L. H . , F. F. Peterson , and R. B . Grossman, 1966, Morphological and
__
genetic sequence of carbonate ac cumulation in desert soils: Soil Sci., v . 1 0 1 , p. 347-360. Harrison, R. S . , 1977, Caliche profiles, indicators of near-surface subaerial diagenesis, Barbados, West Indies: Canadian Petroleum Geol. Bull., v. 25 , p. 123-173. James, N. P., 1972, Holocene and Pleis tocene calcareous crust (caliche) pro files; criteria for subaerial exposure:
Subaerial Exposure
51
F:giiC6 a2- Pia rt tu:a (Tra•d31 tirl(i}. Qf o�?..,., errta! facl�g stoC\"le ot ()r.e i:lt: �;�;: PiM.'\ !:!<.li:r.li:"lg, t-: oustnr f·Jot� ;XIIOJ•�
::;x>nge
tex� rc. 3tQne :;.tr; ki'10'"'l"'l ·
Jour. Sed. Petrology, v. 42, p. 81 7-836. Klappa, C. F . , 1 978, Biolithogenesis of Microcodium; elucidation: Sedimen tology, v. 25, p. 489-522. ___, 1 979, Calcified filaments in Qua ternary calcretes; organa-mineral in teractions in the subaerial vadose en vironment : Jour. Sed . Petrology, v. 49, p. 955-968. __, 1 980a, Brecciation textures and tepee structures in Quaternary calcrete (caliche) profiles from eastern Spain; the plant factor in their formation: Geol . Jour. , v. 1 5 , p. 8 1 -89.
, 1 980b, Rhizoliths in terrestrial carbonates; classification, recognition, genesis and significance: Sedimen tology, v. 27, p. 6 1 3 -629. ___, in press, A process-response __
model for the formation of pedogenic calcretes: Jour. Geol. Soc. London . Knox, G. J . , 1977, Caliche profile forma
tion , Salkhana Bay (South Africa): Sedimentology, v. 24, p. 657-674. Krumbein , W. E . , 1 968, Geomicrobiology and geochemistry of the " Nari-Lime Crust" (Israel), in G. Muller and G. M . Friedman , eds. , Recent developments in carbonate sedimentology in central Europe: Berlin, Springer-Verlag Pub . , p. 138- 147. __ and C. Giele, 1979, Calcification in a coccoid cyanobacterium associated with the formation of desert stromatolites: Sedimentology, v . 26, p . 593-604. Perkins, R. D . , 1 977, Depositional frame-work of Pleistocene rocks in
South Florida: Geol. Soc. America Mem. 147, p. 1 3 1 - 1 98 . Price, W . A . , 1925, Caliche and pseudo anticlines: AAPG Bull. , v. 9, p . 1009- 1017. Read, J . F . , 1 974, Calcrete deposits and Quaternary sediments, Edel Province, Shark Bay, Western Australia, in Evolution and diagenesis of Quaternary carbonate sequences, Shark Bay, Western Australia: AAPG Mem. 22, p . 250-280. ___, 1 976, Calcretes and their distinc tion from stromatolites, in M. R. Walter, ed. , Stromatolites, developments in sedimentology 20: Amsterdam, Elsevier Sci. Pub . , p . 55-7 1 . Reeves, C. C . , 1 970, Origin, classification and geologic history of caliche on the southern High Plains, Texas and eastern New Mexico: Jour. Geology, v . 78, p. 352-362. ___, 1976, Caliche; origin, classifica tion, morphology and uses: Lubbock, Texas , Estacada Brooks, 233 p. Ruellan, A., 1967, Individualisation et accumulation du calcaire dans 1es sols et Ies depots quaternaires du Maroc: Cah. Off. Rech . Sci . Tech . , Outre-Mer , Ser. Pedal . , v. 5, p. 421-460. Swineford, A., A. B. Leonard, and J. C . Frye, 1 958, Petrology of the Pliocene pisolitic limestone in the Great Plains: Kansas Geol. Survey Bull . , v. 1 30, p. 97-1 1 6. Teruggi, M. E . , and R. R. Andreiss, 1 97 1 , Micromorphological recognition
Or: gin and as.)(:!
cr raci r .�
of paleosolic features in sediments and sedimentary rocks, in D. H . Yaalon, ed . , Paleopedology; origin, nature and dating of paleosols: Jerusalem, Int. Soc . Soil Sci. and Israel Univ. Press, p. 1 6 1 - 172. Ward, W. C . , 1 975, Petrology and diagenesis of carbonate eolianites of northeastern Yucatan Peninsula, Mex ico, in K. F. Wantland and W. C. Pusey, eds . , Belize shelf-carbonate sediments, clastic sediments, and ecology: AAPG Stud. in Geology No. 2, p. 500-57 1 . __ R . L . Folk, and J . L . Wilson, 1 970, Blackening of eolianite and caliche adjacent to saline lakes, Isla Mujeres, Quintana Roo , Mexico: Jour. Sed . Petrology, v. 40, p. 548-555.
Watts, N. L., 1 977, Pseudo-anticlines and other structures in some calcretes of Botswana and South Africa: Earth Sur face Processes, v. 2, p. 63-74. ___, 1 978, Displacive calcite; evidence from recent and ancient calcretes: Geology, v. 6, p. 699-703 . __ , 1980, Quaternary pedogenic cal cretes from the Kalahari (southern Africa); mineralogy, genesis and diagenesis: Sedimentology, v. 27, p. 661 -686. Yaalon, D. H . , ed . , 1 97 1 , Paleopedology; origin, nature and dating of paleosols: Jerusalem, Int. Soc. Soil Sci. and Israel Univ. Press, 350 p .
52
Subaerial Exposure
S·J- ·!ant tuf.:� . Se<.: ti
fiE; t: r e-
dont
b;ocky calcite cement:.'lllon .
�RI�X I{nP. .
P.�rrolr.r.a. :!p:lin. 1 1" 11'1 �ec ::; M, �lane pola r l:red lo'�ht �ale .t-ar D.5 mm .
Located
above
caliche I '
-
Bignot , G . , 1 974, Le paleokarst eocene d'lstrie (Italie et Yougoslavie) et son in fluence sur Ia sedimentat ion ancienne : Memoires et Documents, 1 974 nouvelle scrie, v. 1 5 , Phenomenes Karstiques, tome II, p. 1 77- 1 85. Carannante , G., V. Ferreri, and L . Si
Ann. Soc . Geol. Belg . , v. 93, p. 247-254. R ead , J. F., and G. A. Grover, Jr . , 1 977, Sc alloped and planar erosion surfaces, Middle Ordovician limestone, Virginia; analogues of Holocene exposed karst or t i dal rock platfor ms : Jour. Sed. Petrology, v. 47, p. 956-972. Somerville, I. D . , 1 979, A cyclici ty in the
mone, 1 974, Le cavita paleocarsiche cretaciche di Dragoni (Campania): Boll.
early Brig anti an (D2) limestones east of the Clwydian Range, North Wales and
Examples of Possible Fossil Karst Facies
Soc. Nat. Napoli, v. 83, p. l - 1 1 . D'Argenio , B . , 1967, Geologia del grup po Taburno-Camposauro (Appennino Campana): Soc. Naz. Sci. Fis. Mat . , (3), v . 6 , p. l-2 1 8 . Gilewska, S . , 1 964, Fossil karst in Poland : Erdkunde, v. 1 8 , p. 1 24- 1 35. Jordan, G. F . , 1954, Large sinkholes in Straits of Florida: AAPG Bull . , v. 38, p . 1 8 1 0- 1 8 1 7 . Kobluk, D. R . , e t al, 1 977, The Silurian Devonian disconformity in southern Ontario: Canadian Petroleum Geol . Bull . , v. 25, p. 1 1 57- 1 186. Malloy, R . J., an d R. J. Hurley, 1 970, Geomorphology and geologic structure; Straits of Florida: Geol. Soc. America Bull . , v. 8 1 , p. 1947-1972. Mullins, H. T., and A. C. Neumann , 1 979, Geology of the Miami Terrace and its paleo-oceanographic implica tions: Marine Geology, v. 30, p. 205-232. Pirlet , H . , 1 970, L'influence d 'un karst sous-jacent sur Ia sedimentation calcaire et !'interet de !'etude des paleokarsts:
detritico-carbonatiche dell'altopiano di Campumari (Sardegna sud-occidentale): Soc. Geol. ltalia Boll . , v. 95, p. 1 52 1 - 1 540. Fol k , R . L . , and E. F. Mc Bride, 1 976, Possible pedogenic origin of Lig urian ophicalcite; a Mesozoic calichified serpentinite: Geology, v. 4, p. 327-332. Harrison, R. S., and R . P . Steinen, 1 978, Subaerial crusts, caliche profiles, and breccia horizons; comparison of some
its u se in correlation : Geol. Jour . , v . 1 4 , p . 69-86.
Holocene and Mississippian exposure surfaces, Barbados and Kentucky:
Van der Lingen, G. J . , D. Smale, and D .
Geol . Soc. America Bull . , v. 89, p. 385-396. Hubert, J. F . , 1978, Paleosol caliche in the New Haven Arkose, Newark Group, Connecticut: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 24, p. 1 5 1 - 1 68. Marzo, M., M . Esteban, and L . Pomar,
W . Lewis, 1 978, Alteration o f a pelagic chalk below a paleokarst surface, Ox ford, South Island, New Zealand: Sed. Geol . , v. 2 1 , p. 45-66. Walken, G. M . , 1 974, Palaeokarstic surfaces in Upper Visean (Car boniferous) limestones of the Der byshire Block, England: Jour. Sed . Petrology, v. 44, p. 1232- 1 247 .
Examples of Possible Fossil Caliche Facies
1 974, Presencia de caliche fosil en el Bu ntsandstei n del valle del Congost (provincia de Barcelona) : Acta Geol. Hispanica, v. 9, p. 33-36.
Allen , J. R. L . , 1 974, Sedimentology of the Old Red Sandstone (Siluro
Nagtegaal , P. J . C., 1969, Microtexture in recent and fossil caliche: Leidse Geol. Meded . , v. 42, p. 1 3 1 - 142.
Devonian) in the Clee Hills area, Shropshire, England: Sed. Geol ogy , v. 1 2, p. 73- 1 67. Burgess, I . C., I% 1 , Fossil soils of the upper Old Red Sandstone of south Ayr shire: Trans. Geol . Soc. Glasgow, v. 24, p . 1 38- 1 53. Cocozza, T. , an d A . Gand in , 1976, Eta' e signi ficate ambientale delle facies
Roper, H. -P. , and P . Rothe, 1 975, Pe trology of a fossil duricrust ; the " Kerneoldolomit-Horizont", Permian, S. W. Ger many (abs . ) : Nice, France, 9th Int. Sediment Cong . , v. 2, p. 1 0. Steel, R . J . , 1 974, Cornstone (fossil caliche) - its origin, stratigraphic, and sedimentological importance in the New Red Sandst one, western Scotland : Jour.
Subaerial Exposure
53
Hgl;�e :M-�e:egosne {coniat.)li!<:} /,r�s;or. i to ; o.mif1;:tted encru�tatiol"l o.·1
Jurassi0 ca: bona;�s. fn0ru�t�t km::. in th.$ lOCiAdt')" a re uc ;c � c rro {0.4 : n) tt' ick
and occur in sa il sp 1 ay f! r.d s plash zon>3:s Tanag0r:a coasr:ln-3, Spai;'l.
ct too
Geology, v. 82, p. 35 1 -369. Walls, R. A., W. B. Harris, and W. E . Nunan , 1 975, Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky: Sedimentology, v. 22, p . 4 1 7-440. Winchester, P. 0 . , 1 972 , Caliche-like limestones in the Lower Permian Laborcita Formation of the Sacramento
Mountains, New Mexico (abs .): Geol. Soc. America Abs. with Programs, v . 4, p. 707.
Economic Considerations and Case Histories Bernard, A. J., 1976, Metallogenic pro cesses of intra-karstic sedimentation, in G. C. Amstutz and A. J. Bernard, eds . , Ores in sediments: Berlin, Springer Verlag Pub . , p. 43-57. Brown, J . S . , 1 970, Mississippi valley-type lead-zinc ores: Mineral. Deposita, v. 5, p. 1 03- l l 9. Carlisle, D., 1 978, Characteristics and origins of uranium-bearing calcretes in Western Australia and South West Africa (abs .): Jerusalem, Israel, l Oth Int. Sediment. Cong . , v. I , p. 1 1 9 . Chenoweth, P . A . , 1 972, Unconformity traps, in R. E. King, ed . , Stratigraphic oil and gas fields - classification, ex ploration methods, and case histories: AAPG Mem. 1 6 , p. 42-46. Collins, J. A . , and L. Smith , 1 975, Zinc deposits related to diagenesis and in trakarstic sedimentation in the Lower
Ordovician St. George Formation, western Newfoundland : Canadian Petroleum Geol. Bull . , v. 23, p. 393-427. Enos, P., 1977, Tamabra limestone of the Poza Rica trend, Cretaceous, Mexico , in H. E. Cook and P. Enos, eds. , Deep-water carbonate environments: SEPM Spec. Pub. 25, p . 273-3 14. Geldsetzer, H., 1 976, Syngenetic dolomitization and sulphide mineraliza tion, in G. C. Amstutz and A. J. Ber nard, eds . , Ores in sediments: Berlin, Springer-Verlag Pub . , p. 1 1 5- 1 27. Nicolas, J . , and P. Bildgen, 1 979, Rela tions between the location of the karst bauxites in the northern Hemisphere, the global tectonics and the climatic
variations during geological time: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 28, p. 205-239. Padalino, G . , et a!, 1 976, Ore deposition in karst formations with examples from Sardinia, in G. C. Amstutz and A. J . Bernard, eds . , Ores in sediments: Berlin , Springer-Verlag Pub . , p. 209-220. Valeton, I . , 1972, Bauxites developments in soil science 1: Amster dam, Elsevier Sci. Pub., 226 p . Walters, R . F., 1 946, Buried pre Cambrian hills in northeastern Barton County, central Kansas : AAPG Bull . , v . 30, p. 660-710.
Other References Cited in Text Assereto, R. L . A . M . , and C. G . St. C. Kendall , 1 977, Nature, origin and classification of peritidal tepee struc tures and related breccias: Sedimen tology, v. 24, p. 1 53 - 21 0 . Bathurst, R. G. C . , 1975, Carbonate sediments and their diagenesis;
developments in sedimentology 12: Amsterdam, Elsevier Sci . Pub . , 658 p.
Choquette, P. W . , and L. C . Pray, 1 970, Geologic nomenclature and classifica tion of porosity in sedimentary car bonates: AAPG Bull . , v. 54, p . 207-250. Dunham, R. J . , 1 969a, Early vadose silt in Townsend mound (reef), New Mexico, in G. M. Friedman, ed. , Depositional environments i n carbonate rocks; a symposium: SEPM Spec . Pub. 14, p. 139- 1 8 1 . __ , 1 969b , Vadose pisolite in the the Capitan Reef (Permian), New Mex ico and Texas, in G. M . Friedman, ed. , Depositional environments in carbonate rocks; a symposium: SEPM Spec . Pub. 14, p . 1 82- 1 9 1 . Freytet , P . , 1973, Petrography and paleo environment of continental carbonate deposits with particular reference to the Upper Cretaceous and Lower Eocene of Languedoc (southern France) : Sed . Geol . , v. 10, p. 25-60 . Lindstrom , M., 1 963 , Sedimentary folds and the development of limestone in an Early Ordovician sea: Sedimentology, v. 2, p. 243-275.
54
Subaerial Exposure
Purser, B. H . , and J. P . Loreau, 1 973, Aragonitic , supratidal encrustation on the Trucial Coast , Persian Gulf, in B. H. Purser, ed., The Persian Gulf: New York, Springer-Verlag Pub . , p. 343-376.
Schlager, W., and N. P. James, 1 978, Low-magnesian calcite limestones form ing at the deep-sea floor, Tongue of the Ocean, Bahamas: Sedimentology, v. 25, p. 675-702. Scholle, P. A . , and D. J. J. Kinsman, 1974, Aragonitic and high-magnesian
calcite caliche from the Persian Gulf - a modern analog for the Per mian of Texas and New Mexico: Jour. Sed. Petrology, v. 44, p. 904-9 16. Shinn, E. A., 1969, Submarine lithifica tion of Holocene carbonate sediments in the Persian Gulf: Sedimentology, v. 12, p. 1 09-144. Sloss, L. L., 1 979, Global sea level change; a view from the craton, in J. S. Watkins, L . Montadert, and P . W. Dickerson, eds. , Geological and geophysical investigations of continental
margins: AAPG Mem. 29, p. 461 -467. Steinen, R . P . , 1 974, Phreatic and vadose diagenetic modification of Pleistocene limestone; petrographic observations from subsurface of Barbados, West In dies: AAPG Bull . , v. 58, p. 1 008- 1 024. Vail, P . R . , R. M. Mitchum, Jr. , and S. Thompson, I I I , 1 977, Global cycles of relative changes of sea level, in C. E . Payton, ed. , Seismic stratigraphy - ap plications to hydrocarbon exploration : AAPG Mem. 26, p. 49-2 12.
Subaerial Exposure
55
Lead-Zinc Deposits of Bleiberg-Kreuth Thilo Bechstadt Barbara Dohler-H irner
L
ead and zinc ores of the Alpine Triassic occur main ly within shallow water sediments of lower Carnian age; these sequences underwent emer sion periods and were affected by weathering and meteoric karstifica tion (Lagny, 1 975; Bechstadt , 1 975a, b, 1 979; Assereto et al, 1 976). The Bleiberg-Kreuth mineralization within the Wetterstein limestone is controlled by at least four geologic factors : ( 1 ) it occurs on lagoonal plat forms, situated some distance from the mainland to the north, which might have been the original source of the metal; (2) on these platforms it is localized within areas where an ex tensive cavity network had been formed by karstic weathering; (3) the mineralization is bound to areas close to or with peritidal cyclic and evaporitic sedimentation ("special facies" of Schneider, 1 964) ; and (4) it occurs below sealing shales (Raibl beds). Only the subaerial exposure facies is described in more detail , whereas the conflicting theories concerning lead-zinc mineralization are mention ed only briefly (Figs. 1 , 2). Sediment Types
The different facies types can be partly related to water depths in peritidal areas . The first order sedi ment type is an often mud-supported, yellowish, Megalodont-bearing limestone. These lamellibranchs, together with locally abundant dasyclad algae show the original sub tidal deposition of the rock. Solution fabrics, together with vadose cements, however, indicate postsedimentary changes in water depth . Arenitic-ruditic limestone, often with small laminated fenestrae, con-
LAOOIAN
F:gurc 'I · Pn!cogoogr3phic croos sec· Sle�erg·Kfeu::1
lions an� :1:� of the .=�re8 .
taining abundant tepee-structures and tepee-breccias (Assereto and Kendall, 1 977) are placed in the intertidal en vironment. Milky-white dolomitic limestones resp . Dolomites with frequent laminated fenestrae, prism and sheet cracks, flat pebble conglomerates and various vadose cements (Bechstadt,
1 974) hint to environments near and above the mean sea level (inter- to supratidal). Black breccias and greenish marly carbonates (often called "green marls") are interpreted as subaerial exposure facies (Fig. 3). The black to Copyright@ t 9S3 by The American Association of Petroleum Geologists.
56
Subaerial Exposure
§
brown breccia components can either be found within the green matrix or within brownish mudstones with fenestral fabrics (inter- to supratidal), the fenestral fabrics usually on top of the mudstones . The flabby figure and small cavities within the components point to carbonate solution which took place under a meteoric regime, as indicated by the vadose cements within the cavities. The black color is caused by pyrite and bitumen im pregnation of the components. They are discolored normal Wetterstein limestones of mainly the subtidal type (Fig. 4). Associated inter- to supratidal "milky" components are not discolored, possibly due to early lithification. Because of the common fragile appenditures of the black brec cia components they could not have been transported a greater distance in their present condition .
LEGEND
r 1 +
Basin
g
The green marly layers frequently
show erosional features at their base, cut and fill structures, or are restricted to cavities and fissures or dikes within the un derl ying mostly subtidal rock. Carbonate solution is indicated by the irregular size of the cavities, up to half a meter or even more in size . Megalodont-shells were dissolved as well, the former shells partly filled by the green matrix. Statistical investigations (Bechstadt, 1 975b) of the transition frequencies between the a.m. sediment types show a first order association of the green layers with inter- to supratidal sediments (with breccia components) on their top, followed frequently by thicker inter- resp . supratidal rocks. Because subtidal rocks are the domi nant facies in the Bleiberg cyclothems, a tuffitic origin (assumed by some authors) is unlikely as no coincidence between inter- to supratidal conditions and volcanism can be expected.
Volconics
,
Suboerial expowre
( q Cj J EJ D • .. deposition fE!:J � SM� Stumping
Conglom&rOtic: foyer$ Shoftow watef' (:orbonQ!es
Non
Sandston& strWks
P.g u.·e 2 -Nort� �outh st:atlgrapl'llc sec · ti011 ;.., the Ble! ber� .(('c•.lth cJ I 0 .<-l (lr:caticil'1 is !i-re
same as
Fig 1).
1he
c roos
sec11on i11
Subaerial Exposure
57
3-()r�:� "�Ni:ls' cc.mti.llnin;; � dol<:-millc (btJ1 n�t diSC·�Io�ad) ¢0mpo· ncnl, � rl.ain !ly ir)ter�IJPr�!'i(bl , m ilk>' �1or.1ite witt: Jan n�a\ed (a·taot:t�e. fi9ure
Fig �.. re G reco .;; c<Jim�t ·,.,;m :oklck �cia. �npone:1ts. Note :,e:r .freg;lar CO!To�:J r.ent5.
The same association of sediment types is known from other stratigraphic levels inside and outside the Alps-for example, the Lofer cyclothems of the Norian Dachstein limestone. Cyclic emersions of lagoonal platforms are assumed here as well (see Fischer, 1 964; Zankl, 197 1 ) . Barthel and Seyfried (in press) describe other occurrences from the
stratigraphic record as well as recent black breccias (Figs. 5, 6) . Early Diagenesis Meniscus cement and vadose pisolites are indicative of a vadose in fluence . Dripstone cement, indicative of inter- to supratidal resp . vadose environments can be found within cavities of subtidal rocks (Fig. 7).
This indicates a relative sea-level fall shortly after sedimentation. This type of cement is also common within the black breccia-components and the inter- to supratidal rock types themselves. All of these cements are frequently dolomitized, partly together with in ternal sediments. This dolomitization of the former aragonite is thought to
58
Subaerial Exposure
.-rtertt &lprot-4Q1 �..�.
�ro
�C.1 dDI)
G1'91!t�� eo� ��
:tlatll !lracd:l ��!1jl011blll'.;
With
$Jbtid�l
�-lde�>li.t.�
fo=l4Jt;r�
�!11a bli
few m ete:-s 111 ; ck:
I�Uf� 6-�Ctlell'IC'lfiC proliles with: n thP. Bls iberg · Krsuth rn�. 011y :;. f{Nt o1 �he ('ioM·�·j C')'CI�S Within �h� "sp!<:t8. ra�ies · ·
•
CKl .Qrf!dllbr�:notl!o 1-?-] �oted �lff611'Jt3 [6] TbpQo �1Mo"Me m 8� ble
[Z] .�t��
J
L..----- -
Subaerial Exposure
59
fl\:llJ' 8 7-Dr,;.,>sTO;')e -ce:n�1"' on top ,,r a. Cl'l'.iJty wilnin iniN·SUptatidal OC ICm�e.
Stage 2 ; Mineralisation F�g ure 6-Fissu� ·Nithin su b�tdai rock, �ilicd wirrl gr.�en !';ed!ma"'t Sm�ll <:avitles at the left of the big fi ssu re �m pari�J bu rrows and �rtly <.n8$0IVE-d �IOOO!ll shells flllecl wil t> g�n · · m � rl$."
StQge 1: cavern formation
Figur� 9-� )'potr. et cal mocei vf �.•er1\.S :ea�ing to r11i r. era l �tion. St� c ., � of IO'Ner Ca M i <m S19'2; whe t �<;; SW-ge 2 :.$
probably pos�·CaMian.
eo
SUbaerial
Exposure
TABLE 1 1
NazO
MgO
AI203 Si02
(% ) (%) (%)
P20s
(%) (%)
K20 GaO
(%) (%)
li02
MnO
(%) (%)
(%) number (n)
Fe20s
(ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Cr (ppm) Cu (ppm) Co (ppm) Ni ( pp m ) Sr (ppm) number (n) v
Rb Zr Ba Zn Pb
Table 1 . Mean content of selected elements within the subtidal (1 ), Intertidal (2), inter- to supratidal (3), and subaerial exposu re facies (marly carbonates) (4).
C Uelow l�l l�(
I
3
2
4
0.35
0.33
0.23
1 . 09
1 . 82
6.52
9.6
1 .02
1 .0 1
1 . 06
2.4 4.93
0.41
1 . 35
1 . 42
1 .1 9
0. 1 1
0.09
0 . 07
0. 1 2
0.21
0.23
0. 1 8
0.64
53.31
5 1 . 06
46. 1 5
40.37
0.06
0.04
0.02
0. 1 4
0. 1 3
0. 1 3
0.09
0.1 1
0.21
0.21
0.25
0.30
58
39
'
9
8
9.51
1 2 .64
22. 1 4
31 .7
1 6 .00
1 8.02
1 8.78
36.2
3.01
5. 03
1 2 .57
1 8.0
63.49
55.64
73.45
1 1 1 .1
88.96
72 . 5 5
30.83
1 29.25
1 0. 1 4
1 0. 84
9.83
1 0 .01
1 4. 44
1 4.22
1 4.35
30. 1 3
6.02
7.73
5 .23
1 0 .58
1 1 .23
1 0 .97
8.58 1 0 .6
2.60
2.43
0.66
1 5.8
1 37 . 47
1 42.43
220.92
1 79 . 0
55
47
10
10
t-'lg. u e 10-Snll\ll (:f � f{.""'!'l C�!COO:! I I l ,
s �r. at tl•e =Jwer rf:� ht of lhe plc ! U re. M �a'l1ti�3 ar$ partty �1;"1'1 e:11·!:11 ed
?.IICi nmstly !f=1"le r'31il'-ad . Aml'.vs d :- lguro3o 10 af1d , 1 IJ<'�nf to \:1e Sd'ne r.aV: tv.
be indicative of a relative sea-level
fall as well. A recent analog was described from Quaternary limestones of Fuerteventura, Canary Islands (Muller and Tietz. 1971). Megalodont shells are commonly
dissolved . Geopetal green sediment, radiaxial fibrous calcite, and drusy sparite (but no vadose cements) can be observed within the former shell. The country rock must have been cemented at the time of the shell
dissolution because there is almost no alteration of the former shell outline (Fig. 8).
Geochemistry
The geochemical data are based on
Subaerial Exposure
61
F; g,Jtf: 1 1 -De�:..=3.il Clf Fl!lu r� 1 o ;..rr�>'li :s lhe s1ni� lYle ts� .n figure W. and �lnts to a s�� lma nt-flll9d I.)�V. t l". a11c thP.I'I to a sme:l spMlerlte mi:·wai:zar:on ;d tbe top. :t1e .t.igge� cavity ct:ows a II I'J\ !lCncrati on IJ( lllOOtly C!lett: ice: IY r recipiti=l�C:: �h.E\Ier:;e. fo!loo.-ieci by �akma ar:d oerr. er;( as sec�6 Md th. r<1
qene-�tions. two sections within the Bleiberg mine. Zinc , lead, copper, chromium, cobalt and nickel were measured by AAS , while the other elements of Table I were measured by x-ray fluorescence (Dohler-Hirner, in prep. ) . A com parison of the subtidal type with the inter- to supratidal type shows positive/negative correlations of some elements, with the intertidal values placed in between (Table 2). Most of these results are confirmed by investigations of four other sec tions within Drau Range and the Northern Limestone Alps. The high strontium values of the supratidal type are unique to the Bleiberg section. After the results of Ferguson and others ( 1975), zinc-enrichments should be expected at first sight within the supratidal type. The positive correlation with the subtidal sediments might be due to diagenetic
TABLE 2 Element Subtidal Inter- to Supratidal
+
+
+
mobilizations, responsible for the zinc sulfides. However, extreme zinc anomalies, not seen in the averages of Table 1 , are usually found within either the inter- to supratidal type or the green layers . The green, marly carbonates show high values of most elements, being partly constituent of or fixed to clay minerals . These clays consist (nearly entirely) of illite (Krumm, personal commun . , 1 980). Only one sample contained traces of montmorillonite.
+
+
+
-
+
+
+
+
Depositional Environment
A restricted , partly evaporitic en vironment is indicated by the local occurrence of anhydrite and the limited fossil content. The dasyclad algae Poikiloporel/a duplicata and C/ypeina besici are indicative of pro tected lagoonal environments some distance from a central reef (Ott ,
1 967 , 1 972) . The black breccia components and the cavity filling green layers, inter preted ao; reworked paleosoils, are
62
Subaerial Exposure
FiQu r e · 2 (;aJitt int� r · p:.:�le\i a s l\;�rit1c:. wilh� " · · UIN)SI�ne blocK" <>� Kreult: area . Tr.e <:avi�· 1� fl'le-j ,i'h dO!Omitic CO-'npJ(Ie1 mu;�tl\f sphal0. 1 ilc Arrov� -:.! i=fHI.!�e3 · 2 &rlC ! 3 PO'·'lt to lh0 same cc nop::.nen �.
related to oscillations in sea level. Within such peritidal lagoonal areas , sea-level falls of a small scale will result in emersion areas . The eustatic control of the cycles is favored because of the excellent correlation of the sections within the Bleiberg mine in an east-west distance of at least 7 km (43 mi) . However, the facies to the north and south differ. Most of the beds deposited im mediately before the subaerial ex posure (especially inter- to supratidal beds) were stripped away, reworked , and partly dissolved. Meteoric water filled small depressions and caused vadose diagenesis. The dolomitization of the vadose cements is thought to have taken place later, when the sea level started to rise again (evaporative pumping model). This was also the time of anhydrite formation. H 2S and organic-rich layers were formed in small pools below the sur face. They impregnated many of the breccia components, mainly along small fissures, causing a black color . The result of a further sea-level rise is a relatively uniform distribution of the former paleosoils. The green matrix often fills only cavities and small dikes within the underlying rock, whereas the black breccia com ponents can also be found several centimeters above, within inter- to
supratidal sediments. This redeposi tion of the former paleosoils is thought to be the explanation for their intercalation within the subtidal rock. Other parts of the mine underwent longer periods of erosion, karstifica tion and dolomitization, due to tec tonic events. Karst cavities in these areas can be highly mineralized if they are covered by sealing shales . Other areas underwent karstification even during the following Raibl time. In the westernmost part of the mine an area of unmineralized dikes within the Wetterstein limestone is partly filled with Raibl marls and dark Raibl dolomites . Mineralization
Locally present depositional fabrics within the lead-zinc ores, originally thought to represent external sedimentary deposition (Schneider, 1 964; Schulz, 1 964) are now explained as internal cavity-fillings. Dzulynski and Sass-Gustkiewicz ( 1 977, 1980) believe in ore-bearing solutions caus ing the synchronous formation of (hydrothermal) karst structures and the emplacement of the ores . The mineralized cavities are assumed to be preferentially located nearly beneath ancient land surfaces, some being earlier meteoric karst cavities (Fig. 9).
Bechstadt ( 1 975a, 1 979) thinks of a meteoric karst as caused partly by sea level changes, and partly by tec tonism . In this model the relation be tween mineralization and "special facies " is easier to understand . The metals might originate from an ero sional area, assumed to be in the north . Lead isotopes show an age of about 350 m.y. (Carboniferous) for most Alpine lead-zinc deposits (Kop pel and Schroll , 1 978) . An ascending transport of leached metals from beneath is unlikely, as the Car boniferous in most Alpine regions is represented by an unconformity. The enrichment process might be due to the evaporitic conditions ex isting on parts of the lagoonal plat forms. Lead-zinc concentrations of a factor of 200 to 300 within unstable mineral phases are known from ex periments by Ferguson and others ( 1 975). Even today the zinc maxima occur within the inter- and supratidal rock and the paleosoils. Lead and zinc freed from the unstable minerals, might have been transported by (hot or cold) pore fluids, using karst cavities as an aquifer . Sulfides may have been formed where metal-rich and organic sulfur-rich brines were mixing (Figs . 10, 1 1 , 12, 1 3) .
Subaerial
Exposure
>-igu:�; 1 3-09tail SELECTED REFERENCES
Assereto, R . , et al, 1976, Italian ore/ mineral deposits related to emersion surfaces-a summary: Mineral. Deposita (Berl .), v. 1 1 , p. 170- 1 79. , and C. G. Kendall, 1977, Nature, origin and classification of peritidal tepee structures and related breccias: Sedimentology, v. 24, p. 153-210. Bechstadt, T., 1974, Sind stromatac tis und radiaxial-fibroser Ca1cit Faziesindikatoren? : N. Jb . Geol. Palaont . Mh., v. 1 974, p. 643-663. , 1975a, Lead-zinc ores depen dent on cyclic sedimentation (Wetters tein limestone of Bleiberg-Kreuth, Carinthia, Austria): Mineral. Deposita (Berl.), v. 10, p. 234-248. , 1 975b, Zyklische sedimenta tion im erzfuhrenden Wettersteinkalk von Bleiberg-Kreuth (Karnten, Oster reich): N. Jb. Geol. Palaont . Abh . , v . 149, p. 73-95. , 1979, The lead-zinc deposit of Bleiberg-Kreuth (Carinthia, Austria); palinspastic situation, paleogeography and ore mineralzation: Verb . Geol. B.-A., v. 1978/3, p. 221-235 (Proceed. 3rd ISMIDA, p. 47-61). Dohler-Hirner, B., in prep., Unter suchungen zur mikrofazies und geochemie des oberen Wettersteinkalkes (Nord1iche Kalka1pen, Drauzuc): Univ. Munich, West Germany, Doctoral thesis. Dzulynski, S., and M. Sass-Gust__
__
__
__
kiewicz, 1 977, Comments on the genesis of the eastern Alpine Zn-Pb deposits: Mineral. Deposita (Berl.), v. 12, p. 219-233.
and , 1980, Dominant ore forming processes in the Cracow Silesian and eastern Alpine zinc-lead deposits: Proc. 5th Quadrennial IAGOD Symp., p. 415-429. Ferguson, J . , B. Bubela, and P.-J. Davies, 1975, Simulation of sedimen tary ore-forming processes; concentra tion of Pb and Zn from brines into organic and Fe-bearing carbonate sediments: Geol. Resch ., v. 64/3, p. 767-782. Fischer, A. G., 1974, The Lofer cyclothems of the Alpine Triassic, in D. F. Merriam, ed., Symposium on cyclic sedimentation: Kansas Geol. Surv. Bull., v. 169, p. 107-149. Koppel, U., and E . Schroll, 1978, Bleiisotopenzusammensetzung von Bleierzen aus dem Mesozoikum der Ostalpen: Verb. Geol. B.-A. , v. 1978, p. 403-409. Lagny, P., 1975, Le gisement plom bo-zincifere de Salafossa (Alpes italien nes orientales); remplissage d'un paleokarst triasique par des sediments sulfures: Mineral. Deposita (Berl.), v. 10, p. 345-361 . Muller, G., and G . Tietz, 197 1 , Dol omites replacing "cement A" in biocalcarenites from Fuerteventura, Canary Islands, Spain, in 0. P . Bricker, ed . , Carbonate cements: Johns Hopkins Univ. Stud. Geology, v. 19, p.
__
__
63
of F� l.. i o 12.
327-329. Ott, E . , 1967, Segmentierte Kalkschwamme (Sphinctozoa) nus der alpinen Mitteltrias und ihre Bedeutung als Riffbildner im Wettersteinkalk: Bayer, Akad. Wiss. , math.-naturw. Kl . , Abh . , N . F . 1 3 1 , 96 p. , 1972, Die Kalkalgen-Chronol ogie der alpinen Mitteltrias in Angleichung an die Ammoniten Chronologie: N. Jb. Geol. Palaont. Abh. , v. 1 4 1 , p. 8 1 - 1 1 5. Schneider, H. -J., 1964, Facies differ entiation and controlling factors for the depositional lead-zinc concentration in the Ladinian geosyncline of the Eastern Alps: Devel. Sedimentology, v. 2, p . 29-45 Schulz, 0., 1964, Lead-zinc deposits in the Calcareous Alps as an example of submarine hydrothermal formation of mineral deposits: Devel. Sedimen tology, v. 2, p. 47-52. Zankl, H., 1 97 1 , Upper Triassic car bonate facies in the Northern Limestone Alps, in G. Muller, ed., Sedimentology of parts of central Europe, p. 147- 185.
__
.
64
Subaerial Exposure
Travertines Ramon J u l i a
T
ravertines are accumula tions of calcium carbonate in springs (karstic, hydro thermal) , small river s , and swamps, formed mainly b y in crustation (cement precipitation and/or biochemical precipitation). The term travertine has a local origin from Tivertino, the old Roman name of Tivoli in Italy where traver tine forms an extensive deposit. It has already been used by Lyell , 1 863 ; Cohn, 1 864; Weed, 1 889; and Howe, 1 932. These deposits have also been reported as tufa, calc tufa, calcareous tufa, plant-tufa, carbonate concre tions, petrified moss, Vaucheria tufa, Fir.ure 1 -�and Gampl� frcm a ® m Chironomid tufa, spring-sinter, snow'ing .:l iffcrcnl ;"J'' OtJ)"'l�lr,gtca.l CD.IC: L!rll calcic-sinter, sinter crust , and others , c..:-bo:'laC'9 �.ut1ac�s accoodtn;:t to ve�ta! oommu:'liliGs. m m�se:1 !CtllfortDtJI(Jl? and by local names . The term tufa (::ltf!fllll IHh.o.rr.) H = t;erlH r:�� (Psil:::r refers to highly porous, spongy (IJ.NJrurJiana) ::: - �'lo.o'lCfl•�.;l&.e. deposits, and was already used by Plinius (thophus) for incrustation on vegetal remains and porous volcanic rocks. According to Pia ( 1 933), the term sinter should be restricted to lower plants, but most commonly on those deposits of an abiotic origin algae (blue-green , green}, mosses, that are typically more dense and hepatics (Fig. l) and on insect larva compact than tufa; most probably caparace (Chironomids) (Fig . 2), and sinter-crust includes flowstones and in some cases they may be considered other speleothems. as stromatolites. In order to avoid semantic confu Published works on recent and sion, the present trend is to designate subrecent travertines are numerous, dating from the 1 9th and 20th cen as travertine all the carbonate in crustation on plant remains (in place turies. Among the most remarkable and debris) without reference to the research works were those carried out pore volume or density. As a matter by the German school; Pia ( 1 933-34), Wallner (1933-35}, Stirn ( 1 964), Irion of fact , many fossil travertines are and Muller ( 1 968}, and others. They very dense because of complete cementation of all the originally point out the linkage between the dif abundant cavities. ferent vegetal genera and the forma Travertines can form on higher and tion of travertines which shows a morphology according to the in Copyright� 1983 by The American Association of Petroleum Geologists. crusted or incrusting vegetation. -
Recently, more specific problems have been studied . From a petrologic point of view Malesani and Vannucci ( 1 975) studied the precipitation of calcite-aragonite and the diagenetic transformations. Couteaux ( 1 969), Marker ( 1 973), Adolphe and Rofes ( 1 973), Wiefel and Wiefel { 1 974), Schnitzer ( 1 974), Jacobon and Usdoski ( 1 975), and Geurts ( 1 976) en visaged the genetic perspective and studied the correlation between the physico-chemical and biochemical parameters of algae and organisms in volved in the processes of travertine formation. These works show two main trends: ( 1 ) The predominance of physico-chemical processes over the biochemical ones; the main
Subaerial Exposure
65
F; !)UI t! '? -·1 rl'l o.. E:' i.' l .. t;t;·lt by �ntwocrr:ldae l;;t€-8. Ttl€se tiJtl�s 3 t e made li P � e>i.ter'1a: ps�uoo!'�.dfal c�.lc;�e c�t&ls (50 �m) ovc� a!'l ir.�o rn tube or rn:crc�.rysts. i! i ne C31Cite . tQI.RSTIC f>AOCES:31!S I I •
:
I
I I I
•
I
r
•
•
• •
1 \!�<.00C\. OJr.r� l. A (.� 1'0<)1.
parameters being the evaporation, the turbulence and the temperature; and (2) the predominance of biochemical processes over the physicochemical ones. The vegetal activity (photosyn thesis) regulates the rate of C02 in the water and hence indirectly the calcite precipitation . Moreover the vegetal activity may precipitate calcite within the living vegetal cells (Geurts, 1 976).
01>14'\
'11·� I �.
I I
I
�
Geological Setting
Precipitation of calcium carbonate requires that the water reach favorable concentrations of C02 and Ca + + . The element most subject to change is C02 , which is controlled physically by Henry's Law (relation of C02 with temperature and pressure) and biochemically control led throughout photosynthesis (Fig . 3).
1-�guo � D i.:lg·a:-n o:l(:p.oung GMCC(l t:'a
t:on
cr ca:clum tons ar:Cl c�Hb«i a:o>::(le fa ... c ratllf:l t-o t�e formation ot ua ...� r tinas. I'Jote th.:<1 t.:�c1ws ,'lfit:cti.'1g ��� bon -:!iox.
id� oor:(.;er:rraticn inozvde �!leial ;J rovl it • arY.i Mbd?.nce i!'l if!� w� lef b•)dy.
In order to have precipitation of calcium carbonate it is necessary to have a decrease of C02 in water. The processes of physico-chemical and/or biochemical incrustation occur mainly: (a) in karstic (Fig. 4) or ther momineral springs (Fig. 5), where the vegetation and/or temperature changes (mainly cooling of thermal water reaching the atmosphere) favored the calcium carbonate
66
Subaerial Exposure
f: Qur� .tl -ltenica sheets :r.a: n:y ;:;wt cy in�rustatlcr of mcssea :n a t;ar c e:?
�p.r!� (t;oll"��. U cid
Figu•e 5-Tra\'ertine
:;ccumi.J:�ticr: for:'1'1 i:1g pocls aJl� darns �ainl'f �c a lga. activity in Mam�oth Hot Spri.nr; {Yc:ltrl':'3tonej. On� cw1 see di rfwcr 1t
dus
wowth s�·lee: horizontal sheets in P:lOI&. vc.'�iot!l to sub'o�cf.ic.:JI r...'"l��tc In dr.m� a11cl w;�·,�J inc:lj{�d � ���ts �vnen sem: llttl>3 C'.aSCadEI$
precipitation . For that reason the travertines have some characteristics inherent in the karst (for example long prismatic crystals resembling many speleothems) due to physico chemical precipitation of calcium car bonate; (b) in small stream channels that usually show a characteristic pat tern; they are shaped by several natural dams and pools (rimstone dams and rimstone pools) (Figs. 6, 7).
The dams are mainly formed by in crustation of vegetal communities (Fig . 1) (related with water level) and sometimes begin by interruption of the flow due to leaf, stem and trunk fall (Fig. 8). In the pools (always with lower and higher plants) detritic sedimentation processes coexist with incrustation processes . The clasts are mainly formed by fragments of dams destroyed by floods and by oncoliths;
and (c) in swamps and in littoral en vironments of some lakes where the vegetation (lower and higher plants) favors the precipitation processes for ming travertines. In this case the travertines are interstratified with fluvial or lacustrine sediments.
Subaerial Exposure
67
�t£iu:e 6 -�rn� a: u-.: p&;•vl:s bui · � b! �(avertine i n :::l sma li st-eam r
Flg1lfe 7 -·l ra ·..crtiroe aarfiS ar (l �0013 for r-1ac b;• h'fd rot� '3fllla: o•1e -�
GEOMETRIC DISPOSITION
Generally it is possible to distin guish two main forms of occurrence; one in vertical , subvertical and overhanging laminae associated with the incrustation of the vegetation located in the waterfalls and traver tine dams (hanged springs, cascades, etc.) (Fig. 4), and the other in horizontal or subhorizontal laminae
associated with the existence of stream channels (shaped by pool and dams) and paludal environments (Figs . 5, 9). In the first case there exists a mark ed overlap of incrusted laminae and in the second case it is common to find the horizontal superposition of detrital (clastic travertines or not) and bioconstructred travertine beds.
CLASSIFICATION
The classification of the different recent and subrecent travertine deposits has been based on several criteria. One criterion is the physical pro perties, for instance Eisenstuck (195 1 ) classified travertines according to the degree of coherence distinguishing hard travertines and incoherent
68
Subaerial Exposure
: 1gu:e t; · I ia.'lC &-�n1:ple 01 t ravert:!"' v sh(IW:n� diffe r�Ilt vr:,geta: rem3.ins {leaves I 1·1 �ne tor- ar,ct ste'Tls s : r ti'l-3 r:ottom) ltlr.YI!��� by calcium cllrbMil�e. . d:.Je 10 al;;; al ;;��.-11\.':ty rr•1ain l� .�y-9 1'1 0f ic eAA ) Fl-e-c�r;t trs�rtln:za:ion pro· c P.t;Se!> f:Clm Uo!:qw�'s �tre., m . t�:rona.
8pairt.
Figure 9-Pieistocene travertine deposit, with horizontal detrital sheets (d) and vegetal inc rusted remains (v). Bany?les, . Spain .
travertines. Another criterion, this one used by the botanic German school (Wallner, 1 934; Stirn, 1 964) utilizes as a main indicator the type and nature of the plant vegetation involved in the for mation of travertine. This trend has also been used later by Irion and Muller ( 1 968). Other classifications are based on the geomorphological position in which the travertines are formed. In
this way Symoens ( 1 95 1 ) distinguishes spring travertines (denominated after the Belgian term "cron") formed at the foot of hanged springs, and the travertines developed in river beds . The most useful criterion is classification of the travertine as in place and clastic; although commonly there are multiple intergradations. In-place (autochton) travertines are calcium carbonate accumulations on plants in life-position. Clastic traver-
tines are calcium carbonate accumula tions: (a) on debris of higher and lower plants; (b) on clasts constituted by fragments of in-place travertines; (c) on other clasts (lithoclasts, bones, etc.); and (d) on nodular grains, on cholithes, etc. HAND SAMPLES
The most common features of re cent and subrecent hand samples are
Subaerial Exposure
Fig .ue;
!
69
0-Traver1: r: e �rr.1ed b·; .eav e s ir:cr �ta
C0"-'-'11'$(; ::If caJCit;m C,'lrtx>r. :!!te llorts due te> alj,la I �r: vtty {O&:;ilr atoriacea.e;.
1 1 -'rraverline cla ms al'l::1 'XlOI� ,..,,,t: rn,.•· ri.,-, siz� tom1ed t)\.' t.ydrotM r .--.....vll.�"'·inr. wa.er 3.Jld a eae 'CC?· ,... .., ,.,.,..,,.,� {Tur�·J.
the high porosity and morphological variety of the surface covered by calcium carbonate (Figs . 7, 8 , 10). These features are more strongly developed in travertine dam facies and karstic springs. In dams, the water flow accumulates a great amount of vegetal debris rapidly af fected by algal colonization (mainly cyanophyceae), incrusted and, for that reason, immediately incorporated to the dam (Figs. 8, 9) . The vegetal colonization over the dams is quite
.J'e · 2�-l:: t/ ·:i��-, of trol--:e o pc ·t O 'l rcrJsl9d l�af Nota t�e a �al lams rts (a:• ll r :i trc a1 c..i r ;, l -r i c ·o· r i ::·locr :�st�ls •:: f Llv1C (b!; x: 1 9 50.
1 3�3E M vie·.v of & : roken p:)r· . i:.'ln f ·om S.J !; mr:r;c:1 ro: r:·nt alga t ·f.vcr, : ine Th:! .J I;:J::l.l fil:lmt�n.,, (a/ f:�· fl'1 >1 d:-! nsc: net·.vori-; ard :ra o detrilal ;�ralrs �0 = ·:z:. d .,. d atoms, t - hwer1ire . .:1 . ;c 950.
different depending on the level reached by the water. In the areas permanently submerged, the algae (cyanophyceae and clorophyceae) predominate (Fig . lc); in areas af fected by a water level oscillation, the hepaticae (Fig. l h) are more developed, and lastly the mosses (Fig. l m) prevail in the high parts of the dam, commonly recibing the water splash. In the inner part of the pools the clastic facies predominate although
near the dam edges the accumulation of vegetal remains may be important, mainly small stems and leaves . The edges of the pools are colonized by the same vegetation that colonizes the dam (mosses, hepaticae and algae) and vegetation (Miriophilaceae, Charophyte, etc.) develops over the clastic facies of the bottom. In some thermomineral springs where an important algae coloniza tion exists, the hand samples show a typical morphology in steps (Fig. 1 1 )
Figur�
70
Subaerial Exposure
Fig u r e 1 4-Th i n sect ion frorn a t r aver tine formed by leaves (1 ) covered by
fibrous calcite due to algal activity. Re cent travertine of Dosquers stream. Gi rona, Spain; x 70 N . C.
with small pools and dams of cen timetric size. Mineralogy At all times the prevailing mineral is calcite with a low Mg content (LMC) occasionally accompanied by aragonite, mainly in thermomineral water flows (Malesani and Vannucci, 1977) and with near 50Jo content of silica (coming from diatoms, frustules and rhizopoda caparaces) or detritic quartz. FABRICS AND MICROTEXTURES
There are many fabrics related with the different organisms involved in the processes of travertine formation. The vegetation operates in different ways: decaying COz, precipitating biochemical calcite (Fig . 1 2) including metabolic calcite formed in the cells, trapping (Fig. 13) the particles carried by the water flow, and finally serving as substrate to predominant physico chemical precipitation (Fig . 1 4) . From a practical point of view it i s possible t o distinguish two fabrics. One is directly related with algal ac tivity and is characterized by the for mation of micro-cryptocrystalline
anhedral crystals of LMC and microfibers (Fig. 1 2). The second fabric is characterized by the deposi tion of euhedral crystals of LMC over moss stalks, hepatics, and Miriophilaceae, which reach the millimetric range (Fig. 1 5) . The different algal families may show small fabric variations owing to their rhythmic seasonal growth. This seasonal growth , mainly in the Oscillatoriacea, makes up a laminated stratification of millimetric to sub millimetric range (Fig. 1 6) . The spring lamina is the thinner and darker of the two. The micro-cryptocrystalline anhedral crystals as well as the euhedral ones may experience intense neomorphic processes . The long euhedral crystals of LMC can be af fected by degrading processes and can be turned into crypto-microcrystalline anhedral crystals (bio and chemical corrosions). On the other hand the crypto-microcrystalline anhedral crystals covering the algal filaments can be reorganized in such a manner as to form larger crystals of pseudoradial calcite. These neomorphic processes occur very rapidly and this is attributed to the capability of intense water circula-
tion through the highly porous and permeable travertine fabrics. For the same reason, travertine cavities are rapidly cemented and filled up by equant calcite mosaics (Fig. 17) . RECOGNITION I N THE FOSSIL RECORD
Some criteria have been used for recognition of travertine fossils records , the most important are sup ported by related plants and geometric disposition . The travertine bodies are laterally discontinuous and may show subaerial exposure (caliche crusts, karst, etc .) or inter-stratification with fluvial and paludal deposits (lignite beds, marls, siltstones) (Fig. 9). The travertine fossil records also show laminated botryoidal, radial and arborescent carbonate fabrics with an abundance of calcite in mosaics of large crystals (Fig. 1 2) .
Subaerial Exposure
.
71
· .
I .
.. , .., . . ,. . .
·: ,<#
.· ·
.
;
Figure 1 5-Thin section from a t r ave r · tine made u p by cyanoficeae algae. Fibrous c a lcite with long axes of crystals oriented perpendicu lar to vegetal n u cleus; x 70.
Figure 1 6-Trave rtine formed by algal activity (a = Osc/1/atoriaceae) showing the seasonal rhythmic g rowth (dark and white laminae), and by de trital vegetal remains also colonized by algae (v). (Up per Pleistocene, Banyoles, Spain); x 0.8.
Adolphe, J. P . , and G. Rofes, 1973, Les
Tivoli: N . Jb . Min. Geol. Palaont . , v. 32, p. 580-610. Coteaux, M., 1969, Formation et chronol
Levriere (Eure): Bull. A. F. E.Q., v. 2, p. 79-87. Cohn, F., 1 864, Uber die entstehung des
Luxembourg belgo-grand ducal: Bull. A.F.E.Q., v. 3, p. 1 67-1 83. Eisenstuck, M. , 195 1 , Die kalktuffe und
SELECTED REFERENCES
concretionements calcaires de Ia
travertines in den Wasserfallen von
ogie palynologique des tufs calcaires du
ihre molluskenfauna bei schmeiechen
nahe Blaubeuren (Schwabische Alb): N . Jb. Geol. Palaont. Abh ., v . 9 3 , p. 247-276.
Geurts, M. A., 1 976, Genese et strati graphie des travertins de fond de vallee
en Belgique: Acta Geograph . Lo vainesa, v. 1 6, 66 p. Howe, M. A., 1932, The geologic impor-
72
Subaerial Exposure
H;:ll. e 1 7-.$E•! •.·ie'll of a 1ub3 ir ;<, r�· c ;:rt He,�c c�e t '�\'C 'I inc �ho·�vir� :lle ce,. er t t11at nc ���- r.le.<; • "'E' o:: :: ..- :v � l•:':i ':' c :witie� or� p ·odJcod b�· de;
tance of the lime-secreting algae with a description of a new travertine-forming organism : U.S. Geol. Survey Prof. Paper , 170-E, p. 1 9 23 . Irion, G., and G. Muller, 1 968, -
Mineralogy, petrology and chemical
composition of some calcareous tufa from the Schwabische Alb Germany, in G. Muller and G. M. Friedman, eds . , Recent developments in carbonate sedimentology in central Europe: Spr inger Verlag Pub . , p. 1 57- 1 7 1 . Jacobson, R. L., and E. Usdowski, 1975, Geochemical controls on a calcite precipitating spring: Contrib. Mineral . Petrol. , v. 5 1 , p. 65-74. Malesani, P., and S. Vannucci, 1977, Pre cipitazione di calcite e di aragonite dalla acque termominerali in relazione alia genesi e all'evoluzione dei travertini: Atti Acad. Naz. Lincei, R.C. Cl. Sci. F, M.N. Ital, v. 58, p. 761 -776. Manfra, L . , et al, 1 974, Effetti isotopici
nella diagenesi dei travertini: Geol. Romana, v. 13, p. 147- 1 5 5 . Marker, M . E., 197 1 , Waterfall tufas; a facet of karst geomorphology: Zeist . Geomorph . SupBand 1 2, p. 1 38-1 52. , 1973, Tufa formation in the Transvaal, South Africa: Z .
__
Geomorph . , 17-4, p. 460-473 .
Pia, J., 1933, Die rezenten Kalksteine: Tschemarks Min. Petr. Mitt . Erg. Bd. Schnitzer, U. A . , 1 974, Kalkinkrusta tionen und kalksinterknollen in Lias Quellwassern bei Eisenberg (BI. Erlangen-Nord): Geol . B. Nordost Bayen, v. 24, p. 1 88- 1 9 1 . Scoll, D . W. , and W . H . Taft, 1964, Algae contributions to the formation of calcareous tufa, Moro Lake, Califor nia: Jour. Sed. Petrology, v. 34, p. 309-3 19.
Stirn, A., 1 964, Kalktuffworkommen und kalktufftypen der Schwabischen Alb. : Abh. Karst Hohlenkde, E , H . l , 9 1 p.
' Symoens, J . J . , et al, 195 1 , Apercu sur ·la vegetation des tufs calcaires de Ia Belgi que: Bull . Soc. Royale Bot. Belg. , v. 83, p . 239-352. Wallner, J . , 1934, Beitrag zur kenntnis der vaucheriatuffe: Zbl. Bakteriol. Parasitenk. lnfek. , 2. Abt . , v. 90, p . 1 50- 1 54.
, 1934, Uber die bedeutung der sog. chironomidentuffe fur die messung der jahrlichen kalkproduktion durch algen: Hedwigia, v. 74, p. 176-1 80. Weed, W. H . , 1 889, Formation of traver tine and siliceous sinter by vegetation of hot springs: U.S. Geol. Survey, 9th Ann. Rept . , p. 61 9-676. Wiefe1, H . , and J. Wiefel, 1 974, Zusam menhange zwischen verkarstung und travertinbildung un gebeit von weimar: Abh. Zentr . Geol. Inst. Berlin, no. 2 1 ,
__
p . 6 1 -75.
Subaerial Exposure
73
Economic Aspects of Subaerial Carbonates J. Richard Kyle
a!
LIMIT OF PRESOIJ'ILE DOLOSTOOIE NO�THERH LIMIT OF C - HOftiZON
� 5a�����N RI���T S�LE �
'l.?u����RN Ev�':ci�ITEOJ
TRACE Of BASEMENT FAULT
A
n important aspect of sub aerial exposure facies is that karstification and associa ted processes are often re sponsible for the creation of second ary porosity and other features that may become sites for the accumula tion of hydrocarbon or mineral resources , and in the case of some ore deposits, may be responsible for the creation of the valuable commodity. lt is widely acknowledged that subaerial exposure may be an impor tant aspect in the development of sec ondary porosity for hydrocarbon re servoirs (Moore, 1 979). However, the recognition of ancient subaerial ex posure surfaces is often difficult, par ticularly in the subsurface, due to their generally nondiagnostic response to standard geophysical techniques used in the petroleum industry. Generally the effects of exposure, in cluding porosity development or destruction, are more readily ap parent than the exposure surface. The producing strata in many ma jor oil fields are closely associated with unconformities. However, the details of this association often are not well established. The relatively wide spacing of exploration and pro duction wells, i.e. data points, is not generally conducive to reconstruction of the nature of the ancient subaerial exposure surface. The close spacing
of minerals, exploration cores and the three-dimensional view provided by mining exposures permit a far more detailed evaluation of the nature of the exposure surface and the subsur face diagenetic effects. For this reason, two case histories documen-
MAJORGEOLOGIC FEATURES PlNE ?ClNT DISTRICT. NW'T 0
1\tLOMETEAS
--·-;-== ,�
IWJdlfied Bile� Sk!lll ( 1975) Other :ooorcos of Cattl ItOm NOir� (1965) and Pine Point M!n��>
Figure 1 - Maj o r geologic features of the Pine Point district. (Reproduced with modifications from M i n ing Engineering, 1 980, v.
32,
p.
1 6 1 7-1 626.}
ting the economic aspects of subaerial exposure of carbonates were chosen from major mining districts. These ore-hosting carbonate strata are im portant oil-hosting reservoirs elsewhere, and the relationship of subaerial exposure to the development of both types of resource concentra tion is comparable. MINERAL DEPOSITS ASSOCIATED WITH SUBAERIAL EXPOSURE SURFACES
Consideration of the effects of subaerial exposure and weathering on the formation of ore deposits is beyond the scope of this paper as it would include topics as far ranging as supergene sulfide enrichment (por phyry copper deposits), laterite for mation (nickel silicate deposits), and placer concentrations (gold, gem stones, etc.). If the discussion is
restricted to the effects of karstifica tion on sedimentary terranes , then the economic mineral deposits may be divided into four major types (Quinlan, 1 972): ( I ) residual ac cumulations from the removal of soluble strata; (2) deposits preserved in solution-subsidence structures ; (3) subsurface sulfur deposits related to bacterial reactions; and (4) hydrother mal precipitates in solution-produced cavities. Residual accumulations commonly result from the solutional removal of predominantly carbonate strata and include both simple physical concen trations of pre-existing dispersed minerals and chemical reorganization of the constituents of the strata into new minerals. Economic concentra tions of antimony, barite, bauxite, copper, fluorite, gold, iron, lead, Copyright©
1983 by The American Association of Petroleum Geologists.
Subaerial Exposure
74
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Fig u re 2-Cross·section of Pine Point barrier complex with schematic stratig raphic positions of selected orebodies. Note vertical exaggeration of 40X. (Reproduced with modifications from K . H. Wolf, ed., Handbook of Stratabound and Stratiform Ore Deposits: Amsterdam, Elsevier Sci. Pub., v. 9, ch. 1 1 , p . 643·74 1 .)
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that may serve as conduits for fluid migration and sites for ore precipita tion . Deposits of base and precious metals, barite, fluorite, manganese, mercury, uranium, and vanadium have been localized by such features (Quinlan, 1 972). Most investigators feel that the effects of subaerial ex posure are largely separate from the manganese, phosphate, tin, and zinc mineralization event, but Bernard ( 1 973) and Zuffardi ( 1 976) propose have been formed by these mechanisms. Deposits of clay, sand, that mineralization commonly occurs diamonds, coal, and base and as a mature stage of karstification precious metals have been preserved and that the ore constituents may have been released during earlier in subsidence structures related to removal of soluble strata. Karst stages of rock solution. However , related sulfur deposits are associated evidence for the nature of the ore with evaporitic strata in which forming fluids for many deposits is not compatible with derivation from sulfate-reducing bacteria act on hydrocarbons and gypsum or meteoric water at near surface anhydrite to produce hydrogen temperatures (Roedder, 1 976). Con sulfide, calcite, and water. The reduc versely, some authors suggest that ed sulfur is oxidized to elemental solution brecciation is due to the cor sulfur by meteoric water . rosive action of the hydrothermal A significant effect of karstification solutions that formed the ore is the preparation of strata for deposits. mineralization through the develop Karstification appears to have been an important process in the localizament of zones of secondary porosity
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tion of some of the world's major deposits of lead, zinc, and other metals and nonmetallic commodities that occur in carbonate strata of Phanerozoic age. This type of deposit is particularly well developed in the Paleozoic carbonate strata of the midcontinent area of the United States and is often referred to as "Mississippi Valley type" lead-zinc deposits. Similar deposits occur in the Appalachian, Cordilleran, and Arctic Archipelago regions of North America, as well as in Great Britain, central Europe, and northwest Africa (Brown, 1 970). Although there are significant differences among districts, they have a number of uni fying general characteristics (Ohle, 1959, 1980; Sangster, 1 976). Most orebodies occur as stratabound con centrations in shelf carbonate se quences , usually dolostones, on the flanks of major intracratonic basins. The districts commonly occur at facies boundaries of depositional or diagenetic origin, and the mineralized stratigraphic sequence is typically
Subaerial Exposure
75
Figure 3-Hand specimen of high g rade ore consisting of broken col loform sphalerite (s) with skeletal galena (g).
Figure 4-White baroque dolomite in up· per Facies K adjacent to K57 prismatic · orebody. Height of exposure is 1 .5 m .
associated with an unconformity. Mineralogy of the deposits is relative ly simple, generally consisting of large, well-formed crystals of sphalerite, galena, marcasite, pyrite, barite, fluorite, dolomite, calcite, and quartz that are the result of growth in secondary porosity in the carbonate strata. Fluid inclusion data indicate that the mineralizing fluids were ex tremely saline, with temperatures ranging from 60 to 1 75°C (Roedder, 1 976); these mineralizing fluids are similar to the subsurface brines pres ent in many sedimentary basins, for example the Gulf Coast (Carpenter et al, 1 975; Land and Prezbindowski, 1 98 1 ). The ore galenas generally have a wide range in lead isotope composi tion and are often enriched in the radiogenic isotopes (J-type), thus in cticating a complex history. Similarities between oil field brines and inclusion fluids and the spatial association between some hydrocar bon and sulfide accumulations have prompted some to suggest genetic af filiations between hydrocarbons and carbonate-hosted lead-zinc deposits (Anderson, 1 978) . Although there are different opinions concerning metal sources and causes of mineral precipitation, it is now widely ac cepted that these carbonate-hosted ore deposits, like petroleum and
natural gas, are the result of a normal sequence of events in an evolving sedimentary basin (Macqueen, 1979). This "basinal evolution" model for mineralization was first proposed for the Pine Point lead-zinc district in Canada by Beales and Jackson (1966). The Pine Point district in Middle Devonian strata and the Tennessee districts in Lower Ordovician strata provide excellent examples of the ef-
fects of subaerial exposure and con comitant diagenesis on the develop ment of ore-hosting porosity. Fur thermore, the ore-hosting stratigraphic intervals and structures in these ctistricts are important hydrocarbon reservoirs elsewhere. Pine Point Lead-Zinc District
The Pine Point mining district is located on the south shore of the Great Slave Lake in the Northwest
76
Subaerial Exposure
P I N E'
K 57 Fig u re 5-General geologic features of the K57 area. Dots indicate location of diamond drill holes; limestone d r i l l holes indicate areas of upper Pine Point Group l i mestone preserved with i n Facies K . I sopach is detritus thickness i n mete rs . Yellow area represents g reater than 1 OJo total s u l fides over a 53.3 m interval; red area is g reater than 1 OOJo total s u lfides. (Reproduced with modifications from K. H. Wolf, ed ., Handbook of Stratabound and Stratiform Ore Deposits: Amsterdam, Elsevier Sci . Pub. , v. 9, c h . 1 1 , p. 643-74 1 .)
Territories of Canada (Fig. 1). The district has been a major base metal producer since 1 964 and has yielded more than 50 million tons of ore averaging 9 0fo combined lead-zinc; a similar tonnage of ore grade material remains. Orebodies vary considerably in size , geometry, metal percentages and ratios, sulfide textures, and host rock relationships. Individual sulfide concentrations are stratabound in relatively narrow intervals, but orebodies occur in several positions in a 200 m stratigraphic section (Fig . 2). The sulfide bodies are composed almost exclusively of variable amounts of sphalerite, galena, pyrite, and marcasite and associated dolomite and calcite. Individual deposits are zoned with a high grade core of galena and sphalerite (Fig. 3) that grades outward into lower grade material with increasing amounts of iron sulfides toward the perimeter of the sulfide body (Kyle, 1980). Transi tion between high total sulfide material and barren host rock is com monly abrupt (Fig. 4). About 50 sulfide bodies are present within a narrow Middle Devonian carbonate reefal complex which developed in shallow water along a zone of pro longed tectonic instability (Fig. 1 ; Skall, 1 975) . The Pine Point barrier complex was the dominant control of Middle Devonian (Givetian) sedimen tation over a vast area and separated a deep water shale basin (Mackenzie
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Basin) from an extensive evaporite basin (Elk Point Basin) . The Pine Point barrier now dips gently to the southwest from the outcrop belt and hosts several major oil fields in nor thwestern Alberta and scattered smaller gas fields in Alberta, British Columbia, and adjacent Northwest Territories (De Wit et al, 1 973) . Stratigraphy A highly variable sequence of early
Paleozoic evaporitic carbonate and siliciclastic strata up to 100 m thick overlies the Precambrian basement . The early Middle Devonian (Eifelian) Chinchaga formation unconformably overlies the early Paleozoic units and dominantly consists of an evaporite sequence up to 1 1 0 m thick (Norris, 1965). Strata of Givetian age host all known ore deposits in the district. Skall ( 1 975) redefined the Givetian stratigraphy of the Pine Point barrier complex, and facies designated A through P replaced previous forma tional nomenclature (Fig. 2). The ear ly Givetian Keg River Formation (Facies A) conformably overlies the Chinchaga Formation. The Pine Point Group includes the middle Givetian strata (Facies B through K) that form integral parts of the barrier complex (Skall, 1 975). The lower part of the barrier (Fig. 2) consists largely of the fine-crystalline dolostones of Facies B (Off-reef), D (Organic Bar rier), E (Fore-reef), and J (Back-reef)
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and of the bituminous limestones of Facies F (Marine); the evaporite strata of the Muskeg Formation are, in part, lateral equivalents of these units. Dendroid, tabular, and massive stromataporoids, dendroid and massive corals, and brachiopods dominate the framework of the Organic Barrier. The upper part of the barrier consists of limestones of Facies B, C (Fore-reef), D, H and I (Back-reef), the coarse-crystalline Presqu'ile dolostone (Facies K) diagenetically developed from these units, and the Buffalo River Shale of Facies G (Marine) . A partial discon formity separates the upper barrier from the late Givetian Watt Moun tain Formation (Facies L). The Slave Point Formation (Facies M through P) conformably overlies Facies L and is overlain by the Upper Devonian (Frasnian) Hay River Shale. Skall (1975) documented that subtle tec tonic adj ustments along three N65 °E trending "hinge zones" were respon sible for the abrupt shifts in deposi tional environments resulting in the complex facies relationships. Effects of Subaerial Exposure
It appears that karstification and dolomitization were major aspects of carbonate diagenesis related to subaerial exposure which were of great importance in the preparation of fluid-transporting and sulfide hosting structures at Pine Point.
Subaerial Exposure
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Fig ure 6 -Geologic cross-section of the
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K 57
Karstification - The post-middle Givetian, pre-late Givetian partial disconformity is present in the southern and central part of the Pine Point area (Figs . 1 , 2) where the basal late Givetian Watt Mountain Forma tion, Facies L, overlies the coarse crystalline dolostone of the Presqu ' ile Facies K or the undolomitized upper barrier limestones of Facies C, D-2 , H and I . North of the coarse crystalline dolostone development, a considerably thicker Facies L directly overlies Facies B without apparent disconformity (Fig. 2; Skall, 1 975) . The disconformable contact common ly consists of a zone of green clay and carbonate rock fragments overly ing the upper Pine Point Group strata which may contain major green clay in vugs, fractures , and bedding planes for some distance below the' rubble contact . Thickness of the rubble zone is highly variable, apparently depen ding on the local topography on the subaerial exposure surface, but generally is less than 5 m. Major solution features related to this period of subaerial exposure ap pear to control the distribution of sulfides and to account for the geometry of the " prismatic" orebodies with restricted horizontal dimensions relative to vertical extent and of the "tabular" orebodies with restricted vertical extent relative to the horizontal dimensions (Kyle, 1 980, 1 98 1 ) . Within many prismatic
orebodies in the upper Pine Point barrier are strata which are not pres ent in standard Facies K sections, even immediately adjacent to the mineralized zones . Generally this material is light gray, fine- to coarse crystalline, friable dolostone, com monly with a detrital texture. This detritus may be laminated and may contain fragments of green clay and fine-crystalline tidal flat dolostone, particularly in the upper portion. These fragments are generally only a few centimeters in the longest dimen sion but are occasionally as much as a meter. The K57 area illustrates well the nature of the detritus and its relation ship to adjacent strata and to sulfide concentrations of both the prismatic and tabular types . The K57 orebody and the geologically similar K62 orebody occur within the coarse crystalline Presqu'ile dolostone along the Main Hinge Zone in the western part of the district. In this area, Facies K has a maximum preserved thickness of about 45 m and may be divided into upper and lower units. The lower unit consists of tan to light gray, coarse-crystalline dolostone which contains megafossil and uniform granular lithologic types . These rocks are interpreted to repre sent reefal and proximal fore-reef depositional environments, respective ly; a minor amount of this unit is preserved as limestone (Facies D-2).
greater than 2o/o total sulfides; red area is greater than 30% total sulfides. (Reproduced wi th modifications f rom Mining Engineering, 1 980, v. 32, p .
1 6 1 7- 1 626.)
An irregular, interfingering facies boundary between the two lithologic types of the lower Facies K is present just to the south of the mineralized zone (Figs . 5, 6). Direct evidence con cerning the original nature of the up per Facies K unit has been largely obliterated by the pervasive introduc tion of white dolomite. Isolated limestone remnants lateral to this unit suggest that this material was originally the lagoonal Facies H/1. Within the zone of coarse-crystal line dolostone is an irregularly ellip tical area containing light gray, fine to coarse-crystalline, friable dolostone detritus (Figs. 5, 6) . The detritus zone is about 1 90 m long and 1 00 m wide and has a maximum detritus thickness of about 35 m in two areas . Although the top of most of the detritus zone is truncated at its subcrop, Facies L units are present in four drill holes, thus suggesting that the present detritus thickness represents most of the original material . The thickest section of detritus overlies Facies B (Fig. 6) . Near the detritus zone, the contact between Facies B and Facies K is indistinct because the upper part of Facies B is medium- to coarse-crys talline with poorly preserved faunal components . Three stratigraphic units have been recognized in the K57 detritus zone (Fig. 6; Kyle , 1 980). The stratigraphically lowest unit consists of relatively clean, fine- to coarse crystalline dolostone which contains
78
Subaerial Exposure
Figure ! Detritus (dol ine fill) with scat tered tidal flat dolostone (D) and green clay (C) frag ments . K62 o rebody; scale is 54 m m i n diamet e r .
Figure 8-Hand specimen o f detritus " f ragments " (D) cemented by colloform sphalerite and galena. Note isolated col loform sphalerite crusts without substrates and galena vei n l et transac t ing detri tus fragment and its col loform sphalerite ri m .
blocks of coarse-crystalline Facies K dolostone, particularly near the base. A thinner unit of laminated detritus, consisting of alternating bands of light and medium gray dolostone with rare green clay blebs overlies the lower detritus unit. The upper unit consists of nonlaminated detritus with common small fragments of green clay and fine-crystalline dolostone (Figs . 7, 8). Similar dolostone detritus has been observed in at least 10 other prismatic orebodies. Tabular ore-
bodies in the upper barrier complex lack major amounts of associated detritus. Although there are small areas of massive sulfide concentration in these zones, most of the ore con sists of stratabound sulfides confined to a relatively narrow stratigraphic in terval. Tabular sulfide zones are com mon in the lower part of the coarse crystalline Facies K dolostone in some areas; many of the prismatic orebodies have contiguous tabular sulfide concentrations.
M40 is a tabular orebody located in the central portion of the Main Hinge Zone within a cluster of prismatic orebodies . The sulfide concentration in M40 is stratabound in the lower part of the coarse-crystalline Facies K dolostone and averages about 3 m in thickness, although it reaches a max imum of 1 5 m. Most of the ore oc curs as open-space filling of vuggy porosity (Fig. 9), and local collapse breccias also host sulfides (Fig. 10). Although some massive sulfide con-
Subaerial Exposure
79
Figu re 9-Coa rse crystals of sphalerite and galena in tabular porosity zones in the lower Facies K dolostones. M40 orebody; height of exposu re is 1 m.
Figure 1 0-Collapse breccia in lower Facies K dolostone cemented by sph a l erite and galena. M40 orebody; height of exposu re is 2 . 5 m .
centrations are present, sulfide tex tures indicate growth from the walls and suggest that these zones were large voids prior to mineralization. Some of these zones are thin and tabular and contain gravity-controlled stalactitic sulfide forms. Solution features associated with sulfide concentrations within the coarse-crystalline dolostone of the up per barrier differ somewhat from the sulfide-hosting features within the fine-crystalline dolostones of the
lower barrier. The latter sulfide bearing zones occur northeast of the present-day erosional limit of the Presqu'ile (Fig. 1) but presumably were once covered by the coarse crystalline dolostone. Sulfide concen trations may be either massive without lithic components or within breccias and intercrystalline porosity in reefal (Facies D- 1) and back-reef (Facies J) lithologies . N204 is the most northeasterly and the lowest stratigraphically of the presently
known mineralized zones (Fig. 1 ) . Sulfides occur i n the upper part o f a laterally extensive zone of vuggy and intercrystalline porosity and incipient fracturing in the fine-crystalline Facies B dolostones just above the Keg River Formation (Fig. 2). Solution features in the Pine Point barrier complex were of considerable importance in the localization of sulfide concentrations. The detritus zones appear to have been the loci for the deposition of the prismatic
80
Subaerial Exposure
Figure 1 1 -Vadose and ph reatic diagenetic zones in the upper Pine Point barrier during post-Middle Givetian ero sional period. (Reproduced with modifications from M i n i ng Engineering, 1 980, v. 32, p . 1 6 1 7- 1 626.)
Figure 1 2 A-D-Development sequence for sulfide-bea ring det r itus-filled dolines at Pine Point. (Reproduced with modifications from K. H. Wolf, ed . , Hand book of Stratabound and Stratiform Ore Deposits: Amsterdam, Elsevier Sci. Pub., v. 9 , ch. 1 1 , p. 643-74 1 .
A.
250 m
.r y ·,
Fracturing, Brecciollon
� Zone of W: � Calr:itrzotJon ?
Upper
Barriet
l_ ;mestone
- Lower Fine
Border
Dolosrone
Oissolulion
orebodies and contiguous tabular sulfide concentrations in the upper barrier. This association can be demonstrated readily by comparing the distribution of detritus with the total volumetric amount of sulfides. K57 has a maximum total sulfide concentration of about 50 volume percent over a 53 m interval. The greatest sulfide concentration is with in the detritus zone (Fig . 5), but an irregular stratabound zone of lower sulfide content is present to the east of the massive zone (Figs. 5, 6). Most of the sulfides in the stratabound zone occur as pore-filling of the uniform granular lithology (proximal
fore-reeO in the lower unit of the coarse-crystalline Facies K dolostone. Sulfide minerals within the detritus zone fill fractures and breccia in terstices , but large intervals are massive. Despite extensive vuggy porosity within the pervasive white dolomite of the upper Facies K, the boundary of the prismatic orebody is sharp, and the upper Presqu'ile does not contain megascopic sulfides. Minor sulfides occur in intercrystal line porosity in the Facies B dolo stone which underlies the Presqu'ile. Solution features associated with sulfide concentrations in the coarse crystalline Facies K dolostone of the
upper barrier are either large detritus filled depressions open to the discon formable surface or macropores and stratabound zones of increased porosity in the lower Presqu'ile without apparent direct connection with the disconformable surface. These features are thought to have formed during prolonged subaerial exposure with attendant karstification and carbonate diagenesis during post middle Givetian emergence (Kyle, 1 980, 1 98 1 ) . The sulfide-filled macropores i n the lower part of Facies K (for example M40) that lack apparent direct con nection with the disconformity are in terpreted as former caves which developed in the upper part of the phreatic zone during the period of the stable water table (Fig . 1 1 , situation A) . Local collapse in the cave system created breccias which now also host sulfide minerals. Extensive solution of limestone is readily accomplished by meteoric water which has not become saturated with respect to calcite due to C02 loss during seepage of rain water through the vadose zone to the water table (Thrailkill, 1 968). Therefore, macropores and caves developed in the limestone of the up per Pine Point barrier where a bypass permitted vadose flow instead of vadose seepage to the water table (Fig . 1 1 ). The vadose bypasses did not remain open enough to permit
Subaerial Exposure
81
fl. Filll�f!
C. Pre - Sulfide
complete filling of the caves by detritus during the close of the ero sional period. Horizons of increased porosity and permeability were apparently created in the upper part of the phreatic zone adjacent to the zones of major solu tion. This process was responsible for creation of most of the hosting porosity for the tabular sulfide con centrations which are common in the lower Presqu'ile particularly along the Main Hinge. The development of horizons of increased permeability was probably controlled by the local supply of meteoric water with the chemical capacity to produce increas ed porosity. Once meteoric water became saturated with respect to calcite, effective porosity enlargement ceased . This restriction may account for the apparent limited lateral extent of these horizons in the lower Presqu'ile rather than development of a continuous zone of increased permeability along the position of the upper part of the paleo-phreatic zone . The detritus zones are interpreted as filled compound dolines which developed largely by solution activity in the limestone Facies C, D-2, H, and I of the upper barrier (Fig. 1 1 , situation B). Original stratigraphic facies was not the major factor con trolling development of the dolines because they occur in back-reef, reefal, and intercalated facies. The
£mplacomcnl
------ · -
0. Post - Sulfide
Ernplucemenl
-----,
···· ·· ·····" -- ----
82
S ubaerial Exposure
F ig u r e 1 3- Location of mineral districts in the Knox Group relative to major
physiographic and
geologic
'l
provinces in
Tennessee. 1 = Cen t ral Ten nessee Southern Kentucky; 2 Copper Ridge;
3
=
=
Mascot-Jefferson City; 4 = Sweet wate r , (Reproduced with modifications from Economic Geology, 1 976 , v. 7 1 , p . 892-903 .)
dolines are elongate in a northeasterly direction parallel to the Hinge Zones, suggesting that karstification was con trolled by the same structural factors that governed carbonate sedimenta tion. The dolines are as much as 400 m in length and have length to width ratios ranging from about 2 to 3 . 5 . Doline walls were relatively steep and irregular as suggested by the abrupt transition from detritus to wall rock, and depths of at least 35 m are in dicated. In addition to detritus-filled dolines, green clay and carbonate detritus occur in solution-enlarged joints, bedding planes, and vugs near the disconformity. The developmental sequence en visaged for the formation of the sulfide-bearing, detritus-filled dolines is shown in Figure 1 2 . During subaerial exposure, the upper barrier limestones were subject to chemical attack by rainwater, which is under saturated with respect to calcite (Thrailkill, 1 968). Solution was con centrated at joint intersections, generally along the dominant north easterly trend and proceeded until the entire limestone sequence was penetrated (Fig . 12A). Additional solution was slowed at this stage, not only by the underlying chemically more resistant fine-crystalline dolostone, but also by hydrologic fac tors. The position of the meteoric water table may have been controlled
Kno,.; Cen(lzoi c M11sozoic
P«;nnsytvanlan
Ml-sslul pplan Ordovlcian
Group
-
OrdovicianCom brian
100 Kilometers
Combrton
Pr eeombrton
indirectly by sea level on the fore-reef regression. During the final closing of the erosional period, the regolith side of the exposed barrier. Since developed on the karstified barrier most carbonate solution takes place was swept into depressions on the in the upper part of the meteoric phreatic zone (Thrailkill , 1 968) which surface, the largest of which were the dolines (Fig. 1 2B) . This erosional apparently was confined within the material consisted of fine carbonate upper barrier, extensive solution of detritus with some green clay and the fine-crystalline dolostones by tidal flat dolostone fragments eroded meteoric water was greatly inhibited. from the ephemeral tidal flat and The water table eventually became lacustrine deposits of initial transgres relatively stable at a level within the sion. Marine transgression appears to lower part of the limestone units of the upper barrier (Fig. 1 2A). Lateral have been effective in filling the solution of limestone occurred near dolines, and little compaction of detritus appears to have occurred dur the water table adjacent to the ing late Givetian sedimentation (Fig. dolines, and macropores were developed. Slumping of oversteepened 12C) . Collapse of late Givetian strata overlying the detritus zones occurred doline walls resulted in a rubble pile at least in part as the result of volume of large limestone blocks and finer debris on the doline floor . Slumping reduction during sulfide emplacement (Fig. 1 2D). of the doline walls may have effec tively sealed off part of the The relationship of the post-middle macropores so that they were not Givetian episode of karstification to the sulfide-hosting structures in the completely filled with detritus. Following the post-middle Givetian fine-crystalline dolostones of the lower barrier complex is unclear. It erosional period and development of seems unlikely that the zone of most the karstified barrier , marine trans intensive carbonate solution, the up gression first affected the low-lying per phreatic, could have extended in "nearshore" areas, and local tidal to the lower levels of the barrier com flat conditions were established. plex during the post-middle Givetian Periodically, probably during major erosional period . However, carbonate storms, fine carbonate detritus was swept into the standing body of water solution may have occurred in the in the dolines and sedimented in thin zone of mixing of fresh water and sea layers. Inundation of the erosional water which existed within the lower surface was sporadic with several barrier during this period. Another periods of transgression and minor possibility is that the sulfide-hosting
Subaerial Exposure
C E N T RAL
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S CA L E S
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" !I Figure 1 4-General ized stratig raphic sections for the east and central Ten nessee zinc districts. Red bars de l ineate the stratigraphic interval of major zinc concentrations. (Reproduced with modifications from Economic Geology, 1 976, v. 7 1 ' p. 892-903.)
structures in the fine-crystalline dolostones resulted from an earlier period of subaerial exposure and car bonate diagenesis, as Maiklem ( 1 97 1 ) has proposed for the middle Givetian of the Pine Point barrier complex. Dolomitization - All of the Pine Point orebodies occur in dolostones of several types in several deposi tional facies (Fig. 2). The " fine, dense to sandy' ' dolostones contain two distinct types. Some units consist
of uniform, very fine-crystalline (generally less than 20�-t). dense rocks with sedimentary structures and stratigraphic relationships that in dicate deposition in a supratidal to in tertidal environment . Most of the lower Pine Point Group consists of fine-crystalline (20 to 50�-t), dense to sandy dolostones for which stratigraphic, petrologic, and faunal evidence indicates deposition as calcium carbonate sediments in a nor mal marine to slightly restricted sub tidal environment (Fig . 2). Because of the extensive evaporite strata coeval with these dolostones , two major mechanisms for dolomitization have been proposed - seepage reflux of evaporative brines through the barrier
(Skall , 1975), and evaporative pump ing of seawater through the barrier (Kyle, 1 980) . The coarse-crystalline dolostone that occurs between the lower barrier units and the Watt Mountain Forma tion is defined as Facies K (Skall , 1975) (Fig . 2) . The distribution of the coarse-crystalline dolostone is further restricted to the area between the Hinge Zones and below the post middle Givetian disconformable sur face (Figs . 1 , 2). This lithology con sists of dolomite crystals greater than 200�-t in size, and the unit is a diagenetic facies superimposed on back reef, reef, and fore-reef strata (Skall, 1 975). In contrast to the ubi quitous fine-crystalline dolostones of
84
Subaerial Exposure
Figure 1 5 -Cores represen t ing t h e development of l i m estone "equ ivalents : "
intrac lastic pelsparite limestone (a) from the Mascot Dolomite; coarse c 1 ysll:l l l i m� dolostone
(b)
with about 1 0o/o relict
calcite (stained with Alizarin red-S in di lute HCI); and coa rse rock matri x brec cia (c) consisti n g of fragments of chert and shale i n a matr ix of coarse crystalline dolostone.
the lower barrier, the original limestone lithologies are preserved as isolated remnants within the coarse crystalline dolostone and as extensive back-reef strata of Facies H and I (Fig . 2) . Contacts between coarse crystalline dolostone and limestone commonly transect bedding, are ir regular, and are relatively sharp. Relic depositional textures are preserved locally in the coarse crystalline dolostone, particularly in the lower units. White dolomite is a common accessory, and its pervasive introduction has obliterated much of the original nature of the upper part of Facies K in some areas (Fig. 4). The contact of the coarse-crystalline dolostone with the underlying fine crystalline dolostones of the lower barrier is generally abrupt but locally is gradational over a few meters. Four mechanisms have been pro posed for the origin of the coarse crystalline Facies K dolostone: ( I ) dolomitization o f coarse-grained reefal limestones (Norris, 1965) ; (2) recrystallization of fine-crystalline dolostones (Campbell, 1 967); (3) dolomitization of barrier limestones by pre-sulfide heated brines cir culating along the Hinge Zones dur ing post-Givetian times (Skall, 1 975); and {4) dolomitization of barrier limestone related to the mixing of
meteoric water with sea water during the post-middle Givetian erosional period (Kyle, 1 980). The coarse crystalline nature of Facies K does not reflect an original coarse-grained sediment because it is not restricted to the reefal facies and because the coarse-grained reefal lithologies of the lower barrier have not been converted to coarse-crystalline dolostone. The extensive recrystallization of fine crystalline dolostone into Facies K is unlikely because of the thin beds of fine-crystalline Facies J dolostone preserved within the coarse-crystalline dolostone. The coarse-crystalline dolostone is not an alteration effect directly related to sulfide mineraliza tion because it is much more exten sive than sulfide mineralization , is not associated with the orebodies in the lower barrier, and is not present above the disconformity in those orebodies that extend into the Watt Mountain Formation. Both the reflux and evaporative pumping mechanisms can be eliminated as possible methods of generating the coarse-crystalline dolostone because the evaporitic con ditions responsible for the deposition of the Muskeg evaporites and tidal flat lithologies of Facies J had ceased to exist in the Pine Point area by the time of upper barrier sedimentation
(Skall, 1 975). Instead, predominantly lagoonal sedimentation resulted in Facies H and I. The strongest evidence concerning the origin of the coarse-crystalline Facies K dolostone is its distribution below the Watt Mountain disconformity and im mediately "landward" of the most "seaward" extent of the partial ero sional surface (Fig. 2). The disconfor mity is not exposed in the Pine Point area, but the abundant drill hole in formation indicates that the paleo erosional surface is one of relatively low relief with a characteristic karst topography which gently slopes " seaward," generally at less than 1 m/km. Thus, the situation provides a classic model for the dynamic mixing of meteoric water with normal marine water in the subsurface during the post-middle Givetian exposure of the barrier complex. The low relief and lack of major surface drainage on the erosional surface and the apparent lack of stratigraphic restrictions to vadose water flow in the upper bar rier suggest that the water table was not much above sea level. A represen tative figure of 1 m above mean sea level approximately 1 0 km from the point of termination of the partial disconformable surface (that is, the middle Givetian "shore") is reasonable in comparison with the
Subaerial Exposure
85
Figure 1 6- Photomic rograph of pelsparite with g astropod and t r i lobite fragment; the large dolomite rhombs re tain pellet ghosts.
analogous modern hydrologic system of the northern Yucatan Peninsula (Back and Hanshaw, 1 970) . Assuming that the permeability of the upper Pine Point barrier complex was relatively homogeneous during the erosional period , the Ghyben Herzberg principle can be applied (Back and Hanshaw, 1 970) . This principle states that for every unit of fresh water above the mean sea level, the thickness of the fresh water lens floating on salt water of ocean water density is about 40 units. With a hydraulic head of I m, the fresh water lens would have extended within the upper barrier to a depth of about 40 m below the mean level of the post-middle Givetian area. A zone of meteoric water and sea water mix ing would have existed below this depth and could account for the posi tion of the maximum thickness of coarse-crystalline Facies K dolostone about 1 0 km "landward" from "shore" and sloping gently upward to mean sea level . Kyle (1 980) has suggested that the development of the coarse-crystalline Facies K dolostone was related to the dynamic mixing of meteoric water with sea water as an integral part of barrier diagenesis during the post middle Givetian erosional period. This mixing model has been proposed
for dolomitization in several varied carbonate environments (Folk and Land , 1 975); at Pine Point it is con sistent with evidence of paleo geography and other effects of subaerial exposure. The mixing zone was probably rather thin at any par ticular time, but short and long term fluctuations in sea level resulted in migration of the mixing zone and concomitant dolomitization of the en tire area now represented by Facies K (Fig . 2). Post-middle Givetian subaerial ex posure of the Pine Point barrier com plex resulted in the formation of ma jor solution structures and the coarse crystalline Facies K dolostone. These solution features were aquifers for mineralizing fluids and loci for sulfide deposition in the Facies K dolostone. Dolines host prismatic orebodies, whereas caves and tabular permeable zones contain tabular orebodies. Origin of the sulfide hosting breccias in the fine-crystalline dolostones of the lower barrier is less apparent, but these are also believed to be solution features . High-grade sulfide concentrations are localized in dolines and breccia zones because these transgressive features were the bypasses between different aquifers and acted as natural mixing sites for fluids of different character, one of
which contained metals and the other reduced sulfur . Fluid inclusion evidence indicates that the Pine Point sulfides were deposited by highly saline brines at temperatures of 50 to 100°C (Roedder, 1 968). These brines appear to have originated within the sedimentary sequence, but the im mediate metal source cannot be defin ed by present data. Composition of the ores indicates only abundant Pb , Zn, and Fe components in a 2:5:3 ratio . The basinal evolution model for Pine Point (Beales and Jackson, 1 966) states that the metals now pres ent in the orebodies were released by weathering of continental rocks and concentrated into the Mackenzie Basin shales in the form of absorbed metal ions on clay minerals . During compaction and diagenesis of these water-rich sediments, the metals were released to interstitial fluids as soluble metal chloride and organic com plexes. Metal-bearing brines migrated laterally into the permeable reef com plex and travelled in the direction of least hydraulic gradient. Reduction of evaporite sulfates in the Elk Point Basin resulted in concentration of hydrogen sulfide in parts of the car bonate complex. Mixing of metal bearing brines with reduced sulfur resulted in rapid precipitation of metal sulfides. The basinal evolution
86
Subaerial Exposure
•
Oolomite- matria: rubble breccia
�
Fine
•
Ore- matrtK rubble breccia
fB
Coarse
roek- motrhc
rubbJe oreecio
rock- matrix rubble breccia
lJpper KingSIJOr1
t
R bed
_j t
_ _
s bed
I_
_
Figure 1 7-General ized relationships among breccia types, Mascot-Jefferson City district (Reproduced with modifica tions from J . D . Ri dge , ed , Ore Deposits of the United States 1 933- 1 967: Am . l n s t . M in ing , Metal lurgy, and Petroleum Enginee rs , v . 1 , p. 242-256.)
model is very appealing for the geologic setting at Pine Point, and variations of the model have been used to explain the origin of some other carbonate-hosted districts.
Tennessee Zinc Districts Four major mining districts in Ten nessee contain economic mineral con centrations in the upper part of the Knox Group, a 1 ,000 m thick, dominantly shallow water carbonate sequence of Cambro-Ordovician age (Fig s . 1 3 , 1 4) . Ore deposits occur as open-space filling of irregular , strata bound breccia bodies in the Lower Ordovician (Beekmantown) Kingsport and Mascot formations. These strata dip from 10 to 40° to the southeast from surface exposures in the northeast-trending imbricate thrust fault belts of the Valley and Ridge Province in east Tennessee. The Knox Group carbonates in central Ten nessee and southern Kentucky are covered by a minimum of 90 m of nearly horizontal younger strata. The Mascot-Jefferson City and Copper Ridge districts contain orebodies with breccias tightly cemented only by pale yellow sphalerite and dolomite with varying amounts of iron sulfides . The Sweetwater district has long been a significant producer of residual barite
__
. LP.��... U bed
developed from recent weathering of mineralized breccias, but recent sub surface exploration in the area has concentrated on the fluorite potential with byproduct zinc and barite. Reddish-brown sphalerite is the only economic product from the central Tennessee deposits , but the breccias contain local concentrations of galena, barite, fluorite, calcite, dolomite, and quartz. The reader is referred to a special 1 97 1 issue of Economic Geology (v. 66, no. 5) for details on the Tennessee ore deposits . Lower Ordovician carbonate strata in the Appalachians contain numer ous areas with similar mineralized breccias , including economic deposits at Friedensville, Pennsylvania (Callahan, 1 968) , and Daniels Har bour, Newfoundland (Collins and Smith, 1 975) . Furthermore, the abun dant open space still remaining in the breccia systems in some areas of cen tral Tennessee and southern Kentucky results in the upper Knox Group be ing an important aquifer. The brec ciated upper Mascot Formation where capped by relatively impermeable Middle Ordovician strata in a favorable structural position forms a reservoir for hydrocarbon production (Perkins, 1 972).
Stratigraphy The Knox Group carbonates in cen tral Tennessee are underlain by a Cambrian sequence of carbonate and siliciclastic strata up to 500 m thick overlying the Precambrian basement . Although the depositional relation ships are obscured by displacement and crustal shortening in the Valley and Ridge Province, it appears that the pre-Knox sedimentary sequence is as much as 1 ,000 m thick in the area of the mineral districts and increases to the east as the Upper Precam brian - Lower Cambrian siliciclastic wedge thickens into the Appalachian continental margin basin. The Knox Group is comprised of the Upper Cambrian Copper Ridge Dolomite and the Lower Ordovician Chepultepec Dolomite, Kingsport Formation, and Mascot Dolomite (Fig. 14) . These formations are large ly dolostone with some limestone, sandstone, and chert interbeds. Because of the role of stratigraphy in localizing mineralization , the Kingsport and Mascot formations are of particular interest . These forma tions are dominated by sequences of very fine to fine crystalline (less than 50p) dense dolostone beds, many of which are remarkably persistent over thousands of square kilometers and
Subaerial Exposure
87
Figu re 1 8-Recent karst col lapse brec cia in M i ssissippian l i m estones. Putnam
County, Tennessee. Height of exposure is 1 0 m .
have been widely used as marker beds for detailed stratigraphic correlations and structural interpretations. These dolostones seldom have any recognizable faunal constituents and exhibit sedimentary features that are indicative of shallow water deposi tional environments. These "early dolostones" are interbedded with varying amounts of limestone or "limestone equivalents" where these intervals have been diagenetically con verted to coarse crystalline (greater than 200�t) dolostones (Fig. 1 5). Up per Knox limestones are pelmicrites and pelsparites with a sparse fauna dominated by gastropods but also in cluding ostracod, trilobite, brach iopod, and pelmatazoan fragments (Fig . 1 6) . Primary sedimentary struc tures such as shale partings, sand layers, and chert nodules zones can be traced laterally in single exposures from limestone into coarse crystalline dolostone. Economic mineral concen trations often occur in breccias within the transition zone between limestone and coarse crystalline dolostone, hence the importance of the "limestone edge" in minerals explora tion. In east Tennessee the Kingsport Formation consists dominantly of limestone (or equivalents), whereas in central Tennessee approximately equal amounts of limestone and fine crystalline dolostone occur (Fig. 1 4).
The Mascot Dolomite in east Ten nessee is largely fine crystalline dolostone; in central Tennessee, numerous limestone interbeds occur within the Mascot (Fig. 1 4). The top of the Knox Group is a major regional unconformity over much of eastern North America (Har ris, 1971). The post-Beekmantown section is dominantly argillaceous limestones, except for the Devonian Mississippian Chattanooga Shale. A thick carbonate sequence of Mississip pian age forms the Highland Rim provinces, and the Cumberland Plateau consists of Pennsylvanian deltaic sedimentary units (Fig. 1 3) . Effects o f Subaerial Exposure
Unlike the Pine Point district where many of the orebodies are directly associated with the disconformity which represents the period of subaerial exposure that created the ore-hosting zones, the mineralized strata in the Tennessee districts occur considerably below the post-Knox un conformity. The principal ore-bearing zone in the Mascot-Jefferson City, Copper Ridge, and Sweetwater districts is the lower 20 m of the Mascot Dolomite and the upper 50 m of the Kingsport Formation (Fig. 1 4). In central Tennessee the most intense ly mineralized interval is a 60 m sec tion in the middle Mascot, although
significant zinc concentrations occur above and below this interval . The post-Knox regional unconfor mity represents a major period of subaerial exposure. Local relief on the unconformity may be as much as 40 m, and considerable regional beveling has taken place with younger Knox strata eroded to the north and northeast of the mineralized areas. Middle Ordovician strata are thickest where they overlie paleotopographic depressions, and material similar to that immediately above the unconfor mity is present in fractures and brec cias several hundred feet below the top of the Knox. Breccias in the upper Knox Group may be divided into early breccia systems which are generally un mineralized and late breccia systems which host sphalerite and related minerals . Geometry of these collapse breccias consists of a core of rubble breccia surrounded by mosaic and crackle breccia passing outward into undisturbed wall rock (Fig . 17; Hoagland et a! , 1 965 ; McCormick et a!, 1 97 1 ; Hoagland, 1 976; Kyle, 1 976). Both early and late breccias occur in these general forms although early breccias are generally smaller and less well developed. Some in dividual breccia bodies affect tens of meters of section , but most are tabular and confined to thinner
88
Subaerial Exposure
Figure 1 9-Early, fine rock mat rix brec cia with bleached borders on very fin e crystalline dolostone (FD) fragments.
stratigraphic intervals; these geometric relationships can be seen in collapse breccias associated with re cent karstification (Fig. 18). Early, fine rock-matrix breccia con tains fragments of very finely crystalline dolostone in a matrix of pale green, and finely crystalline dolostone with varying amounts of tripolitic chert (Fig . 19). In gross ap pearance some early breccia is similar to the material that immediately overlies the post-Knox unconformity. Fine rock-matrix breccia is present at various intervals within the Mascot Dolomite but is best developed in the upper 1 50 m, that is, near the post Knox unconformity. Early breccia fragments often have bleached rims and single or multiple dark concentric bands or reaction fronts adjacent to unbleached cores (Fig . 1 9) . Reaction rims have been observed in scattered breccia intervals from the top of the Knox to as much as 200 m lower (Kyle, 1976) . Extensive brecciation is not necessary for good development of the rims; otherwise insignificant fractures are often made obvious by bleached borders. Reaction rims are similar to bleaching patterns in iron sulfide mottled dolostones as a result of pres ent-day surface oxidation. This analogy is further supported by the
presence of bleached borders on some fragments of Knox dolostone enclos ed in the basal Middle Ordovician strata. Evidence for formation of reaction rims and early breccias dur ing the post-Knox erosional interval is provided by these associations and by the predominant occurrence of these features in the upper part of the Knox near the unconformity. Early breccias are unmineralized except where transected by later frac tures. This late fracturing often took place without preference to matrix or breccia blocks indicating that early breccias were fairly well lithified by the time of late brecciation. However, much of this subsequent fracturing occurred along early fractures because of the relative weakness of the breccia matrix. It has been suggested that early breccias commonly form the framework upon which the late, ore hosting breccia systems developed in east and central Tennessee (Fig. 1 7 ; Hoagland e t al, 1 965; Hoagland, 1 976) . Late breccia systems in central Ten nessee occupy intervals from above the Knox unconformity to below the top of the Kingsport Formation. The most consistently brecciated and in tensely mineralized interval is the 60 m zone in the middle Mascot . This section is particularly favorable for
formation of tabular collapse breccias related to solution of the thick limestone interbeds. Other intervals may also contain significant intersec tions of mineralized breccia. In east Tennessee, late breccia systems are best developed in the lower 20 m of the Mascot Dolomite and upper 50 m of the Kingsport Formation. Late breccia systems have two types of matrix or cement . The lower part contains coarse rock-matrix breccia which consists of medium and coarse dolomite rhombs that are matrix to differing amounts of very fine to coarse crystalline dolostone blocks, chert fragments , argillaceous material, quartz sand, and silt (Figs . 20, 2 1 , 22). Vague boundaries often occur between coarse crystalline dolostone fragments and coarse matrix, and all variations between clean secondary coarse crystalline dolostone and detrital coarse rock matrix breccia are present in the same stratigraphic position (Fig . 1 5) . Secondary dolomitization and solu tion of limestone appear to be com plementary processes (Kyle, 1 976). Volume-for-volume dolomitization of upper Knox limestone has occurred in some areas of Tennessee. Little col lapse of overlying strata has taken place, and any mineral occurrences are largely restricted to vugs and minor
Subaerial Exposure
89
Figure 20-Coarse rock mat r ix breccia (CR; solution residue) in lower portion of late breccia body in central Tennessee; note reddish brown sphalerite cementing overlying breccia.
fractures. In other areas, solution was apparently the dominant process, and varying amounts of coarse rock-matrix breccia represent the insoluble com ponents of the limestone intervals. In these areas the amount of collapse of overlying units is considerable with either tabular breccia zones developed in selected intervals or occasional "break-through" breccia bodies which affect most of the Mascot section. Displacement of breccia blocks is always downward and may be as much as 1 5 m. In some drill holes in central Tennessee mineralized late breccia systems continue without discontinuity as much as 60 m across the Knox unconformity (Hoagland, . 1 976). The upper part of the late breccia systems may host zinc ore deposits. Sphalerite, galena, fluorite, barite, marcasite, pyrite, calcite, dolomite, and quartz form varying proportions of the cement of the mineral-matrix breccia (Figs . 23, 24). The relative amount of these cementing minerals may change greatly over short distances . All late breccias do not contain economic accumulations of sphalerite. In fact , most are cemented largely by carbonate gangue or still contain abundant open space between fragments. Late breccia fragments do not have
bleached rims like early breccia blocks (Fig. 24), and late breccia blocks with broken reaction rims are occasionally present. These features indicate that bleaching is not directly related to solutions responsible for late brecciation or mineralization and that formation of late breccias took place after cessation of the period of oxidation. The mechanism commonly propos ed for the formation of the late brec cia systems in east Tennessee is enlargement of early breccia zones as the result of limestone solution by mineralizing fluids (Hoagland et al, 1 965) . However , in central Tennessee, sulfides were deposited in equilibrium with early calcite crystals suggesting that the sulfide-depositing fluids could not have been responsible for limestone solution (Kyle, 1 976). It is suggested that the early, fine rock matrix breccia systems represent detritus filled solution structures directly related to the post-Knox ero sional period. Further, solution and dolomitization of limestone beds may be indirectly related to the erosional period . The regional tilting of the Knox strata accompanying uplift and erosion would have resulted in recharge areas capable of supplying meteoric water along permeable zones to strata well below the top of the
Knox . Subsurface solution of limestone would take place during this period. With the termination of the erosional period by marine trans gression, conditions would exist for the establishment of irregular sea water/meteoric water mixing zones. That is, regional topographic highs would remain as recharge areas for the subsurface, while low-lying areas would be inundated by the marine water. With increased meteoric water/marine water mixing (Folk and Land, 1 975), dolomitization would become the dominant process, and dolomitized solution residue (coarse rock matrix breccia) and coarse crystalline secondary dolostones would result . Much of the collapse of the fine crystalline dolostone beds overlying the voids due to solution of limestone probably did not take place until considerably later accompanying loading of the upper Knox Group by Middle Ordovician (and younger?) sediments. Therefore, the ore-hosting collapse breccias may not have developed in their present form until long after the termination of the ero sional period. However, there is in creasing evidence that ephemeral periods of subaerial exposure related to depositional cycles in the upper Knox strata may have played an im portant role in the development of
90
Subaerial Exposure
Fi g ure 22-Photomi c rograph of coarse rock matrix breccia w i th f i n e crystalline dolostone (FD). c he rt (CE), and shale (SH) fragme nts in a coarse crystalline dolostone mat r i x .
F ig u re 2 1 -Hand spec i m e n representing a late breccia system t e Copp er R i dge dist rict . Coarse rock matrix brec cia (CR) ove rlain by a collapse breccia of fine c rys t a l l i n e dolostone fragments (FD) cemented by marcasite (m) a n dolom i te (d).
from h
d
the later breccia system s . Fluid inclusion data indicate that the mineralizing fluids were concen trated brines ranging from 80 to 1 50°C (Roedder, 1 97 1 ) . Hoagland
( 1 976) has proposed that these fluids were derived by sediment compaction in the Appalachian Basin and were transported through the regional Knox paleo-aquifer to the ·mineralized areas. Cementation of the upper part of the late breccia systems by sphalerite and related minerals may have occurred over a considerable period of time. Although di fficult to determine exactly, mineralization may have taken place as early as Middle Ordovician in east Tennessee, whereas a late Paleozoic age for mineraliza tion can be supported for central Tennessee.
CONCLUSIONS The Middle Devonian strata of the Pine Point district and the Lower Or dovician strata of the Tennessee districts provide excellent examples of the effects of subaerial exposure on the development of ore-hosting secon dary porosity in carbonate terranes . The effects of exposure differ somewhat between the areas because of the carbonate depositional en vironments and the nature of subaerial exposure surfaces. Never theless, many of the ore-hosting features such as strata-bound porosity zones in secondary dolostones and collapse breccia bodies are similar. The consistent relationship of carbonate-hosted ore deposits with unconformities and subtle ephemeral exposure surfaces suggests that diagenetic processes related to subaerial exposure may be more im portant than generally recognized in the localization of this important type of ore deposit. Similar processes also produce secondary porosity for im portant hydrocarbon reservoirs.
ACKNOWLEDGMENTS I am grateful for the opportunity to have been involved in exploration and research in the Tennessee and Pine P oint dist rict s and am indebt ed to many in dividuals and mining companies in these
areas for numerous courtesies . Partial support for this publication was provided by the Geology Foundation of The University of Texas at Austin.
SELECTED REFERENCES Karstification and Subaerial Exposure Back , W . , and B. B. H ansh aw 1970, ,
Comparison of chemical hydrogeology of the carbonate peninsulas of Florida and Yucatan: Jour. H ydrology, v. 10, p. 330-368. Esteban, M . , and C . F. Klappa, 1 982, Subaerial exposure surfaces, in P. A. Scholle, ed . , Carbonate depositional en vironments: AAPG Mem . 33, this volume. Folk, R. L . , and L. S. Land, 1 975, Mg/Ca ratio and salinity; two cont rols over crystallization of dolomite: AAPG Bull . , v. 59, p. 60-68 . Mathews, R. K . , 1974, A p rocess ap proach to diagenesis of reefs and reef associated limestones, in L. F. Laporte, ed . , Reefs in time and space: SEPM Spec. Pub. 1 8 , p. 234-256.
Subaerial Exposure
91
Figu re 23 -Col l apse breccia consist ing of fine crystalline dolostone blocks cemented by yellow sphalerite and dolomite . N ote the different dolostone
l i thologies represented by the fragments, ind1cati n 9 stratigraphic disruption ac companying col l apse M ascot -Jefferso n
(;ity dist ri ct ; height of exposure is 4 m.
Moore, C. F . , Jr . , 1 979, Porosity in car bonate rock sequences, in Geology of carbonate porosity: AAPG Continuing Education Course Note Series I I , p . A l -A l 24. Quinlan, J. F. , 1972, Karst-related mineral deposits and possible criteria for the recognition of paleokarsts; a review of preservable characteristics of Holocene and older karst terranes: Sec. 6, 24th InternaL Geol. Cong . , p . 1 56-168. Thrailkill, J. V., 1 968, Chemical and hy drologic factors in the excavation of limestone caves: Geol . Soc. America Bull . , v. 79, p . 1 9-46. Zuffardi, P . , 1976, Karsts and economic mineral deposits, in K. H. Wolf., ed . , Handbook o f strata-bound and stratiform ore deposits: Amsterdam, Elsevier Sci. Pub . , v. 3, p. 175-212.
Carbonate-Hosted Ore Deposits Anderson, G. M . , 1 978, Basinal brines and Mississippi Valley-type ore deposits : Episodes, v. 2, p. 1 5- 1 9 . Bernard, A. J . , 1 973, Metallogenic pro cesses in intra-karstic sedimentation , in G. C. Amstutz and A. J. Bernard, eds . , Ores i n sediments: New York, Springer Verlag Pub . , p . 43-57. Brown, J. S., 1970, Mississippi Valley type lead-zinc ores: Mineral. Deposita, v. 5, p. 103- 1 1 9. Carpenter, A. B . , M. L. Trout , and E. E. Pickett , 1974, Preliminary report on the origin and chemical evolution of lead and zinc-rich oil field brines in central Mississippi : Econ . Geology, v. 69, p .
1 1 91- 1206. Jackson, S. A . , and F. W. Beales, 1 967, An aspect of sedimentary basin evolu tion; the concentration of Mississippi Valley-type ores during late stages of diagenesis: Canadian Petroleum Geols. Bull., v. 1 5 , p. 383-433. Land, L. S., and D . R . Prezbindowski, 1 98 1 , The origin and evolution of saline formation water, Lower Cretaceous carbonates, south-central Texas , U.S.A.: Jour. Hydrology, v. 54, p . 5 1 -74. Macqueen , R. W . , 1979, Base metal de posits in sedimentary rocks; some ap proaches : Geosci. Canada, v. 6, p. 3-9. Ohle, E. L . , 1959, Some considerations in determining the origin of ore deposits of the Mississippi Valley type: Econ. Geology, v. 54, p. 769-789. ___ , 1 980, Some considerations in de termining the origin of ore deposits of the Mississippi Valley type; part I I : Econ. Geology, v . 7 5 , p . 1 6 1 - 1 72. Roedder, E., 1976, Fluid inclusion evi dence on the genesis of ores in sedimen tary and volcanic rocks, in K. H. Wolf, ed . , Handbook of strata-bound and stratiform ore deposits : Amsterdam, Elsevier Sci. Pub . , v. 2, p. 67- 1 10. Sangster, D . F . , 1 976, Carbonate-hosted lead-zinc deposits, in K . H. Wolf, ed . , Handbook o f strata-bound and stratiform ore deposits : Amsterdam, Elsevier Sci . Pub . , v. 6, p. 447-465.
Pine Point Lead-Zinc District
Beales, F. W . , and S. A. Jackson, 1 966, Precipitation of lead-zinc ores in car-
bonate reservoirs as illustrated by Pine Point ore field, Canada: Trans. Inst. Mining Metallurgy, v. 75, p. B278-B285. Campbell, N . , 1 967, Tectonics, reefs, and stratiform lead-zinc deposits of the Pine Point area, Canada, in J. S. Brown, ed . , Genesis of stratiform lead-zinc barite-fluorite deposits in carbonate rocks : Econ . Geology Mon . 3, p. 59-70. De Wit, R . , et a! , 1 973, Tathlina area, District of Mackenzie, in The future petroleum provinces of Canada: Cana dian Soc. Petroleum Geols. Mem. I , p . 1 87-212. Kyle, J. R., 1 980, Controls of lead-zinc mineralization, Pine Point district , Northwest- Territories, Canada: Mining Engineer, v. 32, p. 1617-1626. __, 1 98 1 , Geology of the Pine Point lead-zinc district , in K. H . Wolf, ed . , Handbook o f strata-bound and stratiform ore deposits : Amster dam, Elsevier Sci. Pub . , v. 9, p . 643-74 1 . Maiklem, W . R . , 1 97 1 , Evaporative draw down-a mechanism for water level lowering and diagenesis in the Elk Point basin: Canadian Petroleum Geols. Bull . , v. 1 7 , p. 1 94-233 . Norris, A . W . , 1 % 5 , Stratigraphy of Mid dle Devonian and older Paleozoic rocks of the Great Slave Region, Northwest Territories: Geol. Survey Canada, Mem . 322. Roedder, E., 1 968, Temperature, salinity, and origin of the ore-forming fluids at Pine Point, Northwest Territories, Canada, from fluid inclusion studies:
92
Subaerial Exposure
Figure 24-Hand specimen of a col lapse breccia from central Tennessee
consisting of fine crystalline dolostone blocks (FD) c em e n t ed by sphalerite (s), fl uori te (f), a n d barite (b).
Econ. Geology, v. 63, p. 439-450. Skall, H., 1975, The paleoenvironment of the Pine Point lead-zinc district : Econ. Geology, v. 70, p. 22-45 . Tennessee Zinc Districts
242-256.
Callahan, W. H . , 1968, Geology of the Friedensville zinc mine, Lehigh County, Pennsylvania, in J. D. Ridge, ed., Ore deposits of the United States, 1 93 3 - 1 967: New York, Am. Inst. Min ing, Metal!. Petroleum Engineers, p. 95-107.
Collins, J. A . , and L. Smith, 1 975, Zinc deposits related to diagenesis and in trakarstic sedimentation in the Lower Ordovician St. George Formation, Western Newfoundland: Canadian Petroleum Geols. Bull. , v. 23 , p. 393-427.
Crawford, J., and A. D. Hoagland,
The Mascot-Jefferson City zinc district, Tennessee, in J. D. Ridge, ed. , Ore deposits of the United States, 1 933- 1 967: New York, Am. Inst. Min ing, Metall . Petroleum Engineers, p.
1 968 ,
Harris, L. D . , 1 97 1 , A lower Paleozoic paleoaquifer-the Kingsport Formation and Mascot Dolomite of Tennessee and southwest Virginia: Econ. Geology, v . 66, p . 735-743 .
Hoagland, A. D . , 1976, Appalachian zinc lead deposits, in K . H. Wolf, ed . , Handbook o f strata-bound and stratiform ore deposits: Amsterdam, Elsevier Sci. Pub. , v. 6, p. 495-534. , W. T. Hill, and R. E. Fulweiler, 1965, Genesis of the Ordovician zinc deposits in East Tennessee: Econ. Geology, v. 60, p. 693-714. Kyle, J. R., 1 976, Brecciation, alteration,
__
and mineralization in the Central Ten nesse zinc district: Econ. Geology, v. 7 1 , p. 892-903. McCormick, J. E . , et al, 1 97 1 , Environ ment of the zinc deposits of the Mascot-Jefferson City district, Ten nessee: Econ. Geology, v . 66, p . 757-762.
Perkins, J. H., 1972, Geology and economics of Knox dolomite oil pro duction in Gradyville East Field, Adair County, Kentucky, in Proceedings of the Technical Sessions, Kentucky Oil and Gas Association Annual Meetings, 1 970-7 1 : Geol. Survey, Spec. Pub. 21 , p. 10-25. Roedder, E., 1 97 1 , Fluid-inclusion evidence on the environment of forma tion of mineral deposits of the Southern Appalachian Valley: Econ. Geology, v. 66, p. 777-79 1 .
Subaerial Exposure
93
Paleokarst Development W. Marti nez del Ol mo Mateu Esteban
A
long the margins of the western Mediterranean , Mesozoic carbonates in structurally controlled pa leotopographic highs present a well developed paleokarst of local economic importance with significant oil fields. In the offshore of north eastern Spain , different horizons of the Jurassic-Cretaceous sequences are overlain by Miocene marine sediments, although some low areas contain a basal Paleogene of con tinental facies. Mesozoic series in this area are dominated by carbonate rocks with important dolomitic inter vals . Subaerial exposure and karstification covers a variable time span (40 to 60 m.y.) depending on the location of the paleotopographic highs . Onlapping Miocene sediments (carbonates , shales and sands) date from Lower to Upper Miocene. The Mesozoic-Miocene unconfor mity is marked by up to 100 m of Mesozoic breccia and it is usually dif ficult to establish a sharp limit bet ween Mesozoic and Miocene units. The breccia clasts are highly irregular in size and shape (from large boulder to pebbles) and are cemented by com plex generations of calcite cements and terra-rossa. At the upper part, coastal marine reworking of the brec cia during the Miocene is commonly present. Cavern porosity is well developed as well as calcitization (dedolomitization) and leaching of dolostones. This set of features is in terpreted as indicative of the presence of a subaerial exposure surface. All Copyright© 1983 by The American
Association of Petroleum Geologists.
Fig u re 1 -U nconformity Mesozoic Miocene (green horizon) interpreted as subaerial exposu re su rface , offshore northeastern Spa i n . Yellow reflectors correspond to the Upper Miocene, the l imit Lower-M iddle Miocene is establish ed at the orange reflecto r. Note hog back morphology of the erosion surface.
Figure 2-Four examples of paleokarst development in Mesozoic carbonates, offshore northeastern Spain, with characteristic gamma ray and sonic log response. The subaerial exposure s u r face corresponds to the same event represented in Fig u re 1 .
MARKERS A. Paleosoil 8 . Phreatic level
C.
Base of w h e a t h ered
Masozoic limestone?
D Miocene 1m Breccia [!]] Mesozoic
94
Subaerial Exposure
:1:
!:: ...J
GAMMA RAY
INT. T R A N S IT TIME
MESO Z O I C
S H A LY
40
FACIES
Mark e r s : A . Ter t iar y - m esozo i c u n c o n fo rmity u n k arstified Mesozoic below.
D 11:.[] Figure 3 · - Two examples of the same subaerial exposure surface shown on Figures 1 and 2. but affecting unk.a rstified, shaly M esozoic basement. Logs show no pa rticular response at the unconformity.
seismic profiles in the area (Fig. 1 ) show a marked reflector at the base of the Miocene onlap, locally in angular discordance with the Mesozoic basement . This surface can be easily correlated to nearby land ex posures, where additional evidence of subaerial exposure can be found (Microcodium, rhizoliths, caliche pro files ; see Figs. l la, b, d and g, 68 , and 70).
M i ocene Mesozoic
, 50m. , V er t i c a l S cale
Although non-diagnostic, log response is quite characteristic and useful in local correlations. In the more typical situations (Fig . 2), the gamma-ray log shows a marked in crease at the brecciated horizon . We can distinguish two sharp peaks at the top and the bottom of the brecciated interval (A and B in Fig. 2), charac teristics of clean shale lithology . Usually A is more intense than B,
with other minor peaks irregularly distributed along the brecciated inter val. The sonic log shows a progressive decrease in .6-t from the top to the bottom of the brecciated interval (A-B in Fig. 2), mostly noticeable at the upper part (first 40 to 50 m) . The sonic response also suggests the presence of fractures at and below the breccia. The lower part of the brec ciated interval is characterized by a
Subaerial Exposure
lower but still non-stabilized At . Toward deeper levels, Lit progressive ly disminishes up to a point C (deeper than B) where the response is com pletely stabilized and corresponds to the characteristic of the unaltered car bonate rock formation. This log response can be explained in light of the ideal karst profile des cribed in the text. The top of the pro file (A) shows a highly radioactive
peak , interpreted as a paleosoil and zone of infiltration. The horizon B is probably produced by the top of the phreatic level and its oscillations . This is the zone of major cavern formation and deposition of cave sediments, corroborated by the occurrence of total loss of circulation in the well at this point. The interpretation of the deep interval B-C is less satisfactory, but obviously represents the transition
95
to the unaltered rock; perhaps it coin cides with the transition zone of the ideal karst profile described in the text. Where the subaerial exposure surface occurs on a non-carbonate substrate, log response is quite dif ferent in character (Fig. 3), and pre sents difficulties in the location of Mesozoic-Miocene unconformity.
98
Lacustrine
Upland Forest Figu res 1, 2-Typic a l , moderately deep, north temperate zone lake in late stage of develo�mert. Open lake water is bounded by a floating mat of sedges and reeds form i ng a sedge-reed peat (a). A bog forest (b) accumulating forest peat surrounds and eventually reolaces the sedge mat and is in turn surrounded by dry land and upland torest (c). As lake
Floating
productivity increases, the open lacustrine sediment becomes richer in organic matter and forms an olive to blar.k, fine-grained sediment called gyttja
that may or may not contain carbonate depending upon the chemistry of the lake systern. Such a lake system can produce and preserve sediments that cont<:11n lipid·! ich, herbaceous, and woody o r ganic m atte r .
sediments
S
edimentary rocks deposited in lacustrine systems are known from much of the world, but relatively few have been the focus of exploration for oil and/or gas. Most large oil ant gas fields are related to, or found in, marine strata that are the source or reservoir rocks for hydrocarbons. Large accumulations of oil and gas trapped in rocks believed to have formed in ancient lake systems, however, are known from the western part of the United States , parts of South America and Africa, In donesia, Russia, and from much of China. In addition, shows of hydrocarbons and small oil and gas fields developed in lacustrine strata are known from several other areas . The purpose of this chapter is to il lustrate some of the most important features in lacustrine carbonate rocks, and particularly those that formed in lake basins and have the potential to produce oil and gas. Hydrocarbon source, reservoir, and trap units of Chinese, African, South American and United States fields were deposited in large lacustrine basins comparable in size to inland seas. Some of these lakes existed for
several millions of years, a life span not generally characteristic of most present-day lakes. The ancient lake basins of China contain the principal oil- and gas-bearing lacustrine beds in the world. The Uinta Basin of north eastern Utah is another large lake basin that contains large volumes of hydrocarbons . Unlike the lacustrine strata of China that consist mainly of siliciclastic rocks, those of the Uinta Basin consist of large proportions of both siliciclastic and carbonate rocks. The primary reservoirs for hydrocar bons in the Uinta Basin are fractured sandstone and siltstone, but the hydrocarbon source rocks are car bonate mudstones. Lakes are frequently thought of as
freshwater systems, but are common ly alkaline and/or saline. lt is also evident from analysis of physical, chemical , and biological data that most ancient lakes were dynamic, and the depositional facies reflect frequent changes in their chemistry and bathymetry. As a result , characteristic features used to recognize a particular depositional phase of a lake's history may differ significantly from those of a different lake phase. Thus, the distribution of sedimentary structures and fossils in lacustrine facies can vary significantly from one phase to another as physical and chemical characteristics of the hydrologic basin change. Because of the extreme variability it is very difficult to pro-
Lacustrine
CaCOs sat
Allochthonous sediment
99
All<
me/L
Figure 3-Diagrammatic cross section
of a typical, moderately deep. hard
water, north temperate lake during the
summer st1owing areas ol deposition ol littoral and profunda! carbonate facies,
and subdivision of the lake into epilim
nion, metalimnion. and hypolimnion on the basis of temperature. Profiles of temperature, dissolved oxygen, pH,
CaC03 saturation, and phosphAte ;ue based on data of Megard (1967, 1968).
pose a unique set of the physical, chemical, and biological criteria that specifically characterize lacustrine depositional environments. Many physical, chemical, and
chapter are of rocks that formed in
gas in significant quantities. Ancient lake beds found in the western part of the United States and specifically in Utah, Colorado, and Wyoming have been the focus of many studies of the depositional histories of the rocks. For this reason, and because of the authors' familiarity with these units and of comparable lakes, many
large lacustrine depositional systems
of the materials used in this chapter
biological features of lacustrine rocks
are not unique to this depositional en
vironment . Some illustrations in this and both the lacustrine and associated
are taken from studies of these rocks
peripheral fluvial facies are il
by ourselves and colleagues .
lustrated. Much o f the oil and gas in
the Uinta Basin and China is recovered from lacustrine rocks, but much is also recovered from rocks that formed in environments at the fluctuating margins of the lake or in environments well removed from the
lakes. Hydrocarbons in nonlacustrine beds are believed to have been deriv ed from lacustrine rocks and to have migrated into peripheral faci es . Many descriptions and discussions of a great variety of ancient and
DEPOSITIONAL SETTINGS
Figure 1 is a diagrammatic illustra tion of the distribution of sedimen tary facies resulting from the
postglacial development of a typical north temperate lake , and Figure 2 is a photograph of the surface of such a lake. North temperate lakes com m on ly contain both carbonate and siliciclastic sediments. The physical, chemical, and biological processes in
modern lacustrine depositional
such a Jake are similar in many ways
and many of these papers are listed in
lakes . Because much is known of the
systems are available in the literature, the reference section of this chapter. However, it is evident th at only a few of these were of sufficient size, or
produced and preserved sufficient organic matter, to form oil and/or
to those inferred for some ancient
interrelationships between these pro
cesses in modern lakes , it is helpful to review this information in order to
understand and correctly interpret
facies relations in ancient lacustrine
sequences. The four most important com ponents in the sediments of a typical moderately deep (about 25 m), hard water\ north temperate lake are: (I)
detrital material , (2) biogenic silica,
(3) organic matter, and (4) carbonate
minerals . The relative importance of
each of these components changes as
the lake evolves from a newly filled basin to dry land over a period of about 1 0,000 years. Basal sediments are commonly sand, silt, and clay derived from ero sion of bedrock in the drainage basin,
or transported into the drainage basin
as glacial drift . These contain very lit tle organic matter because the organic
productivity of the lake at this stage is very low (oligotrophic) and there is little vegetation on the surrounding land. If the drainage basin contains
some limestone and/or dolomite, or calcareous glacial drift, leaching of this material produces an accumula tion of dissolved calcium carbonate within the lake. Once the lake water 'Hardwater lakes contain substantial concen
trations (usually more than 1.0 milliequivalents per liter) of total dissolved alkaline-earth ca
tions, mostly calcium and magnesium, and sur face waters are usually saturated with respect
to CaC03 at least during the summer months.
100
Lacustrine
Figure 4-Typ i ca l c rysta l s of low-Mg calcite. Photograph is of a sed iment-trap sample from Green Lake, Fayetteville, New York. Bar = 10p.m.
is saturated with respect to calcium carbonate, one or more carbonate minerals may precipitate and form a carbonate-rich sediment (marl) with varying proportions of clastic material. At the same time that carbonate is leached from the surrounding areas , nutrients are also leached and begin to accumulate in the lake. Organic productivity of the lake increases and more organic debris either is incor porated into the sediment or decays, releasing more nutrients for organic growth . In addition, organic matter is also contributed to the lake from vegetation transported from the drainage basin. The organic matter that accumulates in the lake is a mix ture of both allochthonous material (pollen grains, leaves , needles, seeds, woody material, and other organi c detritus) from the drainage basin, and autochthonous material (largely debris from planktonic algae and aquatic macrophytes). Allochthonous organic matter tends to be enriched in humic, high-molecular-weight, carbon-rich compounds, whereas autochthonous phytoplankton debris is enriched in low-molecular-weight , nitrogen-rich lipids . Studies of sedimentary plant pigments in recent lake sediments indicate that most of the organic matter in moderately to highly productive lakes is autochthonous. Throughout the open lake history of the lake, therefore, the most important source of organic matter is from f loating microscopic algae or phytoplankton. The sediment
Figure 5-Aragonite cement precipitated
subaqueously on micritic aragonite fecal pellets from the brine shrimp Artemia salina from Lake Urmia, Iran. The aragonite in the fecal p e l l ets is precipitated in t he lake during seasonal chemical events. Photograph by Kerry Kelts.
at this highly productive (eutrophic) stage of the lake's development is likely to be a brown or black , lipid rich organic sediment called gyttja. The organic content is usually greater than 2007o (about lOOJo organic car bon) and may be as high as 500oJ (about 250Jo organic carbon) . If the lake is highly eutrophic with a hypolimnion that is anoxic throughout much of the year, or if the lake is permanently stratified (meromictic) with permanently anoxic bottom waters (monimolimnion) that contains hydrogen sulfide and/or methane, the sediment is likely to be a black , foul-smelling ooze called sapropel that contains abundant lipid rich organic matter and ferrous sulfides. As the lake fills and becomes shallower, the littoral zone of rooted aquatic vegetation becomes wider, and a floating mat of sedges begins to grow outward into the lake. The algal gyttja around the margins of the lake is replaced by sedge peat (Fig. 1).
Figure 2 is a photograph of the sur face of a lake at this stage of development. The sedge peat even tually covers the gyttja as the lake continues to become shallower and smaller in area. Once the sedge peat provides a stable substrate, a bog forest can develop and eventually the sedge peat will be covered with a layer of bog-forest peat. The resulting relations among lacustrine sedimen tary facies (Fig. l).would consist of: (1) basal clastic sediments grading up ward into marl as the sediments become increasingly rich in car bonate, (2) marl grading upward into algal gyttja recording increased pro ductivity of the lake, (3) gyttja grading into sedge peat , and (4) sedge peat grading into bog-forest peat as algal organic matter is replaced by herbaceous organic detritus from aquatic and forest vegetation . Such a lake system has the capacity to generate and preserve lipid-rich , her baceous, and woody organic materials that are potential sources of both oil and natural gas. Most temperate lakes are dimictic, characterized by semi-annual over turns, one in the spring and one in the fall. Overturn occurs because water has a maximum density at about 4°C. Therefore, surface water
Lacustrine
Figure 6-Crystals of trona (NaHC03.Na2C03.2H20) from
101
the sur
face deposit of Lake Magad i, Kenya, a saline, sodium-carbonate-bicarbonate lake in the Rift Va lley of East Africa. The
lake is underlain by a deposit of bedded trona up to 50 m thick that has ac cumulated over the past 9,000 years (Eu�sler, 1970; E ugste r and Hardie, 1978). Lake Magadi is fed by alkaline hot springs that create perennial water bodies at the lake margins even during the dry season. Note hammer for scale. Photograph by Hans P. Eugster.
Figure ?-Nahcolite (NaHC03) nodules con s isting of a ggreg ate s of bladed crystals in laminated dol o mitic marlstone (typical oil shale) of the Green River For mation, Piceance Creek Basin. Colorado (Dyni, 1974). Note th at laminae of
marlstone are deformed around the
nahcolite nodules, but some bladed
crystals have grown int o the enclosing marlstone. Photograph by John R. Dyni.
will sink to the bottom when cooled to 4°C in the fall and when warmed to 4°C in the spring. These overturns are the heartbeat of a lake and, in deed , are important in maintaining life in the lake. Overturn is the main way that oxygen is supplied to the bottom waters of a lake. Similarly, during overturns the surface waters are restocked with nutrients that ac cumulate in the bottom waters during summer or winter stagnation . At fall overturn , the entire water column is isothermal at 4°C. Even tually the temperature of the surface water reaches ooc and the lake freezes . The lake can no longer receive oxygen from the atmosphere, and oxygen production by photosyn thesis is at a minimum. Oxygen con sumption by respiration and decay continues and eventually the lake waters may become depleted in dissolved oxygen . Oxygen depletion
occurs first in the bottom waters and moves upward through the winter. If oxygen depletion occurs rapidly, or if ice remains on the lake for a long period of time, the entire water col umn may become deoxygenated, resulting in a winter kill when much of the fish population suffocates. Ice finally melts and the surface water is heated to 4°C. At this time, the lake is isothermal, and the water can be completely mixed by wind, carrying oxygenated surface waters to the bottom and returning nutrients
stored in the bottom waters to the surface . During the spring and sum mer, the surface water is continually warmed, and by June or July a temperature profile of the lake will be similar to that in Figure 3. At this time, the lake water is divided into three water masses by temperature related density differences. The upper water mass or epilimnion may be isothermal as a result of wind mixing. The thickness of the epilimnion is therefore determined largely by the depth of wind mixing. The middle
102
Lacustrine
Figure 8-Laminated kerogenous car bonate mudstone (oil shale) that has been deformed. Man's fir1ger point::; to ca vi ty caused by leaching of water so luble nahcolite nodule to form the bird's nest zone in the saline facies of the Parachute Creek Member of the Green River Formation along Evacuation Creek, eastern Uinta Basin, Utah. Beds are exposed along mined trace of gilsonite vein in the Green River Forma tion (Eocene) near Bonanza, Utah.
Figure
9-lnterbedded halite (deeply eroded layers) and fine-grained to microcrystalline nahcolite from the same horizon in two cores of the Green River Formation separated by about 5 km, Pi ceance Creek Basin, Colorado. Each halite-nahcolite couplet may be a seasonal unit of sedimentation (a varve; Dyni et al, 1970). Unlike diagenetic nodules of nahcolite (Fig. 7), tile nahcolite laminae probably precipitated directly from lacustrine brine (Dyni, 1974). Photograph by John R. Dyni.
water mass (metalimnion) is characterized by a rapid change in the temperature-depth curve (this region of the curve is called the thermocline) . The lower water mass or hypolimnion is usually characteriz ed by a gradual decrease in temperature to the bottom . Depend ing upon the abundance of plant life in the lake and the extent of wind
mixing, the epilimnion may be saturated or even supersaturated with oxygen from the atmosphere and from photosynthesis. In the hypolim nion , however, oxygen may be com pletely eliminated by respiration and decay . The depth at which oxygen production by photosynthesis equals oxygen consumption by respiration and decay is called the compensation
depth. The compensation depth varies daily but is usually in the metalim nion. The part of the lake bottom that is populated by rooted aquatic vegetation is called the littoral zone (Fig . 3). The part of the lake bottom under the hypolimnion is called the profunda! zone. There is usually a transition zone called the sublittoral zone between the littoral and profun-
Lacustrine
103
Figure 1 0-Crystal of ankerite from the water-insoluble residue of bedded
nahcolite and halite in the Green River Formation, Piceance Creek Basin, Col orado. Bar
=
10 JLm. Photograph by
John R . Dyni.
dal zone that is inhabited mostly by algae and mosses. DEPOSITION O F CARBONATE IN LAKES
For simplicity, the sediments of a marl lake can be considered a four component system of carbonate , organic matter, biogenic silica (mostly diatom frustules), and detrital siliciclastics . An increase in the relative abundance of any one of the four components will result in decreases in the relative abundances of the other three . As a result of dilu tion by organic matter, clastic material, and biogenic silica, sediments of most marl lakes, even those known to precipitate large quantities of CaC03 , usually contain less than 50% CaC03 . The major sources of carbonate in lake sediments are: ( 1) inorganically precipitated carbonate, (2) photosynthesis-induced, inorganically precipitated carbonate (hereafter call ed bio-induced carbonate), (3) biogenic carbonate consisting of debris from calcareous plants and animals, and (4) allochthonous (detrital) material derived from car bonate rocks in the drainage basin. Unlike marine sediments in which most carbonate is derived from re mains of calcareous organisms, most of the carbonate in lake sediments is inorganic or bio-induced; however, ostracode- and mollusk-rich layers do
Figure 11-Fossils of the prosobranch gastropod Goniobasis tenera (Hall) preserved in a packstone of the Green River Formation near Soldier Summit, Utah. These gastropods lived in shallow. fresh, oxygenated water at the edge of Lake U inta in the early Eocene. Scale is in centimeters.
occur. Rio-induced precipitation of carbonate is obvious in the littoral zone of a hard-water lake where rooted aquatic vegetation may be covered with a crust of carbonate that may exceed the weight of the plant (Wetzel, 1 975). In the central, open water {pelagic) part of the lake, bio induced precipitation of carbonate is important, but less obvious because the plants are microscopic algae . Mineralogy The most abundant carbonate mineral in both profunda! and littoral sediments of marl lakes is low-Mg calcite , that is, calcite containing less than 5 mole-percent Mg (Fig. 4). Small amounts of aragonite occur, but usually as biogenic aragonite from mollusks. According to Muller and others ( 1972) primary aragonite can usually form in lakes if the Mg to-Ca ratio in the water is greater than about 12. Ratios of Mg-to-Ca
this high are rare in temperate marl lakes but may occur in alkaline saline lakes (Fig. 5). The Mg-to-Ca ratio of the water also determines whether primary high-Mg calcite ( > 5 mole percent Mg) and diagenetic dolomite will form. Observations by Muller and others ( 1 972) indicate that primary high-Mg calcite precipitates when the Mg-to-Ca ratio in the water is between 2 and about 12, and diagenetic dolomite occurs in lakes with high-Mg calcite as a primary car bonate mineral and a Mg-to-Ca ratio between 7 and about 1 2 . Dean and Gorham ( 1 976) reported high-Mg calcite and dolomite in surface sediments of marl lakes in the semiarid prairie regions of western and southern Minnesota. Of the 46 lakes they studied, the lake with the most dolomite in its profunda! sediments (Elk Lake, Grant County, Minnesota) was the only one with a high Mg-to-Ca ratio (7. 7). Dean and Gorham also found that the Mg-to Ca ratio in precipitated calcite in creases with increasing Mg-to-Ca ratio in the lake water. Muller ( 1970) and Muller and Wagner ( 1 978) also found that the Mg-to-Ca ratio of lake water determines the Mg content of
104
Lacustrine
Figure 1 ?-1 aminated kerogenous car bonate that has been desiccated to form polygonal cracks. Cracks are filled with mollusk skeletal fragments including the shells of the gastropod Gonioba.sis sp. These beds are part of the Green River Formation near Soldier Summit in the Uinta Basin. Kerogenous units were formed and preserved in anoxic water ill the fluctuating margin of Lake Uinta. The mollusks probably lived in oxygenated water near the shoreline and skeletal debris filled the cracks during shoreline fluctuations. Scale is in inches and centimeters.
Figure 13-Fossil unionid bivalves sp.) preserved in packstone composed of bivalve and gastropod skeletal debris in the Green River For mation near Soldier Summit, Utah. These strata were deposited in a shallow, nearshore lacustrine environ ment with aquatic plants and fresh. oxy genated water (Fouch et al, 1976). Divi sions on scale are in millimeters.
(Piesielliplio
high-Mg calcite precipitated by exten sive growth of phytoplankton in Lake Balaton, Hungary . Sodium c arbonate and bicarbonate minerals such as trona (NaHC0 3 . Na2C03 .2H20; Fig. 6), nahcolite (NaHC0 3 ; Figs. 7 to 9), and natron (Na2C 03 .IOH 20) are common in sediments of ancient saline alkaline lake systems (C ulbert-
son , 1 966; Hite and Dyni, 1 967; 1 979), which , although rare, have Bradley and Eugster , 1 969; Dyni, been reached in some large modern 1 974; Robb and Smith , 1974; and and ancient lake systems. For exam Eugster and Hardie, 1 975) and ple, saturation with respect to trona modern saline lakes (Eugster and in Lake Magadi , Kenya, is reached Hardie, 1 978; Smith , 1 979). Precipita only after lake waters have been con centrated about 250 times beyond the tion of these minerals requires ex point at which all alkaline-earth car treme brine concentrations of about 300,000 ppm total dissolved solids bonates have been precipitated (Jones and others, 1 977). Iron carbonates (Eugster and Hardie, 1978; Smith ,
Lacustrine
1 05
Figure 14-Calcareous bivalved carapace of a Pliocene ostracode. Bar 100 p.m. Photograph by Richard Forester. =
M.
Figure 15-Living charophyte (Chara sp.) containing about 50o/o dry wei g h t CaC03 (left hand) and carbonate mud that consists mo s tly of low-Mg calcite derived from charophytes (right hand), Green Lake, Fayetteville, New York.
Figure 16-Encrustations of lo w Mg calcite around stems of charophytes, Blue Hole, Ohio. -
siderite (FeC0 3) and ankerite (C a[Mgx Fet -xlC03) are common in some ancient lacustrine strate such as the lower Tertiary Green River For mation (Fig. 10; Smith and Robb, 1 966; Robb and Smith, 1 974; Desborough and Pitman , 1 974; Desborough, 1 978), but with one ex ception they have not been reported from modern lacustrine sediments. The exception is a possible occurrence of siderite reported by Anthony
( 1 977) from varved sediments of Lake of the Clouds, an iron-meromictic lake in northern Minnesota. He found X-ray diffraction lines that correspond approximately to the six strongest lines of siderite. Formation of siderite requires a reducing en vironment and an extremely low sulfide concentration so that ferrous carbonate can form instead of ferrous sulfide (Kelts and Hsu, 1 978). Desborough ( 1 978) reported a large
variety o f Ca-Mg-Fe carbonates in oil shale from the Green River Forma tion. He recognized abundant substitution o f Mg for Fe in siderite to form Mg-siderite, and substitution of Fe for Mg to form ankerite and Ca-ankerite. Smith and Robb ( 1 966) concluded that the dominant car bonate mineral in the oil-rich Mahogany zone of the Green River Formation in the Piceance Creek Basin, Colorado , was ankerite with
106
Lacustrine
Figure 17-Antheridium (a) and oogonium (c). the male and female reproductive structures of Chara sp. The outer covering of the oogonium (gyrogonite) calcifies and is a distinctive, common calcareous fossil in lacustrine carbonate sediments and rocks. Bar= 0.2 mm.
an average composition of Ca(Mg . ssFe . ts)C0 3 and not stoichiometric dolomite. Cole and Picard (1 978) found that dolomite and ankerite are most abundant in the distal , open-lacustrine oil shales in the Parachute Creek Member of the Green R iver Formation in the Pi ceance Creek Basin, Colorado, and eastern Unita Basin, Utah. Manganese carbonate (rhodochrosite) has been reported associated with fer romanganese deposits in Green Bay, Lake Michigan, by Callender and others ( 1 97 3 ) . In the discussions that follow, we generally use the term carbonate to mean collective carbonate minerals in a sediment with a predominance of low-Mg calcite, but with the understanding that some aragonite, high-Mg calcite, and/or dolomite may also be present . We use CaC0 3 in some places, usually in the context of a precipitate in the water, with the implication that the mineral is calcite but possibly has a variable Mg con tent. Additional recent discussions of lacustrine carbonate mineralogy are in Jones and Bowser ( 1 978) and Kelts and Hsu ( 1 978). Profunda! Carbonate
Most of the carbonate that ac cumulates in the sediments of a typical marl lake is generated within the lake. Most detrital sediment that ent er s a lake is deposited in the lit toral zone. Some detrital material re-
Figure 19-Phacotus sp., a planktonic, lacustrine g reen alga from Elk Lake, Clearwater County, Minnesota. This is one of the. few calcareous planktonic organisms that contributes carbonate lo lacustrine sediments. Bar = 1 JtiD.
Figure 18-Calcareous outer cover (gyrogonite) of the female reproductive structure (oogonium) of a late Pleistocene charophyte. Bar = 100 �m. P h o t ograph by Ri c ha rd M. Fo rester.
mains in suspension and is deposited as pelagic sediment in the profunda) zone, but these contributions are usually minor. Redeposited littoral carbonate may be a major source of profunda! carbonate in some lakes, but rarely is it the dominant source. Examples of what we consider to be extreme redeposition of littoral sedi ment occurs in Green and Round
Lakes, Fayetteville, New York (Figs. 22 to 24) . Ludlam ( 1 974) estimated that about 500Jo of the profunda! sediment in Green Lake consists of redeposited littoral sediment transported by turbidity currents (Fig. 22). The sides of Green and Round Lakes are unusually steep (about 30°), however, and much of the lit toral zone is rimmed with overhang ing algal bioherms (Figs. 37, 38; Dean and Eggleston, 1 975 ; Eggleston and Dean , 1 976). Slumping is also com mon on the steep slopes of these two lakes (Figs. 23 , 24). Littoral sediment transported into the profunda! zone by turbidity currents is usually easy to
Lacustrine
1 07
Figure 20-Varve lami n ations from sediments in Elk Lake. Clearwater County, Minnesota. Light layers consist mostly of fine-grained low-Mg calcite and diatoms (Fig. 21); dark layers consist mostly of diatoms and clay. Marks on scale are in millimeters.
Figure 21-Fine·grained low-Mg calcite and dia tom s (Stephanodiscus niagarae) from a varve lamina in sediments from Elk Lake, Minnesota (Fig. 20). Bar = 100 ,,m.
recognize because of differences in color, grain size, textures, bed forms, and presence of littoral plant and animal remains . Fine-grained detrital and resuspended littoral carbonate transported to the center of the lake in suspension and deposited as pro fundal sediment would be difficult to distinguish from authigenic carbonate formed in the epilimnion, but con tributions from these two sources are likely to be very small . The main distinction between pro-
fundal and littoral carbonates is that littoral carbonates contain a larger proportion of biogenic CaC03, most ly debris from mollusks (Figs. 1 1 to 13), ostracodes (Fig. 14), and charophytes (Figs. 1 5 to 1 7); profun da! carbonates rarely contain calcareous organic remains except for small contributions from resuspended littoral material. There are a few freshwater planktonic calcareous organisms such as the coccolith Hymenomonas (Hutchinson, 1 967)
and the green alga Phacotus (Fig. 19; Kelts and Hsu , 1 978), but, in general , the amount of carbonate contributed to profunda! sediments by calcareous organisms is small . This is also the most important distinction between profunda! lacustrine sediments and pelagic deep-sea sediments. If detrital and biogenic contribu tions to carbonate in the profunda! zone are minor, inorganic and bio induced precipitation of CaC03 by removal of C02 becomes the main
108
Lacustrine
Figure 22-Varve laminae and turbidite layers (t) in sediments from G ree n Lake, Fayettevi ll e New York. The light lamina in each varve couplet consists mostly of low-Mg c alcite c rystals precipitated in the surface waters (Fig. 4), and the dark lamina consists most l y of low-Mg calcite, clay and organic matter (Ludlam, 1 969). The light laminae c o ntain an average of about 800Jo ca lcite and l e ss than 1 % ,
organic carbon, whereas the dark laminae contain an average of about 55<1Jo calcite and more than 20'/o or ga n ic
carbon. Most of the calcite is precipitated between May and October (Brunskill, 1 969). Turbidite layers inter calated with normal varve laminations account for about 50<1Jo of the sediment that accumulates on the floor of the main basin of the lake (Ludlam, 1974). Bar 5 em. Photograph by Stuart D. =
Ludlam.
Figure 23-Natural slumping of varved sediments in Green Lake, Fayettevil le, New York, nea r the foot of the steep basin slope. Sed iments of different ages are of different hues. The pal e lamina e in the most recent sediments (post 1950) are cream-colored, and the varves are relatively th ick. The oldest varves (before about 1850) have ol ive hues of gray, tan, and b rown, and the varves are relatively thin. Va rves of intermediate age are gray and of intermediate thickness. The c racks and holes are an artifact of rapid
plankton photosynthesis can be very rapid (more than 1 .0 g C/m2/day in very eutrophic lakes) and can increase the pH to 9.0 or more (Wetzel , 1 975 ; Megard, 1 967, 1 968). In extreme ex amples , C02 may be depleted faster than it can be replaced from the at mosphere, and this depletion favors a d rying. Bar 5 em. Photograph by predominance of phytoplankton that Stuart D. Ludlam. can utilize carbon from bicarbonate as well as from COz for photosyn thesis (Wetzel, 1 975). This increase in pH caused by photosynthetic removal source of profunda! carbonate . of C02 can result in supersaturation Precipitation of CaC03 can and precipitation of CaC0 3 and is theoretically occur as soon as satura the basis of bio-induced carbonate tion is reached, but metastable super precipitation . Bio-induced precipita· saturation of up to several times tion is difficult to prove, because theoretical saturation without without detailed photosynthesis ex precipitation is the rule rather than periments it is difficult to the exception. The most important demonstrate that COz removal is due to photosynthesis and not diurnal factor controlling the C02 budget in most moderately to highly productive temperature fluctuations. The most convincing evidence for hard-water lakes is the balance be tween C02 consumption by bio-induced carbonate precipitation was provided by Megard ( 1 967 , 1 968) photosynthesis and C02 production by respiration and decay. A who investigated primary productivity simplified photosynthesis-respiration in six lakes in Minnesota and at the equation is: same time measured pH, carbonate saturation, alkalinity, and calcium-ion energy + C02 + H 20 � CHz o + Oz. concentration. He observed that there was a linear relation between the rate Rates of C02 removal by phytoat which carbon was fixed by =
Lacustrine
109
F i gure 24-Varve(?) laminae and tur bidite layers in sediments from Round Lake, Fayettevil le, New York. It is not known if the dark and light laminae couplets are strictly annua l accumula tions (varves). Most of the material in both laminated sediment and the ir· regular white mass is composed of discrete calcite crystals similar to those found in sediments from Green Lake (Fig. 4; Ludlam, 1 969), but larger. The white mass appears to be de rived f rom marl deposited around the periphery of the lake at a period of higher lake level and contains n u merous gastropod shells. The turbidites consist mostly of debris from the littoral zone. particularly calcite casts of charophytes and organ ic fragments of other macrophytes These sediments are difficult to sample without disturbance; as a result, some of the structure is an artifact of the sampling procedu re . Bar = 5 em. Photograph by Stua rt D. Ludlam.
Figure 25-Varve laminae from the cen· tral plain of Lake Zurich, Switzerland. Varve couplets generally range between 2 and 5 mm in thickness with 80 to 90% water content. A typical varve cycle con sists of a dark layer containing organic s ludge with algal filaments, iron sulfides, and clay grading upward into a lacy net work of diatom frustules and organic matter, overlain by a light layer contain· ing diatom frustules and calcite at the base and almost pure calcite at the top (Kelts and Hsu, 1978). Varve couplets, episodically interrupted by t u rbidites, have been forming in Lake Zurich since 1895. Bar = 1 em. Photograph by Kerry Kelts.
photosynthesis and the rate at which calcium and alkalinity were depleted from the water, presumably by precipitation of CaC03 . His em pirical results indicate that for these
six lakes an average of four moles of carbon must be removed to precipitate one mole of CaC03 . His empirical results indicate that for these six lakes an average of four
moles of carbon must be removed to precipitate one mole of CaC03 . Some of the more important chemical manifestati ons of phytoplankton photosynthesis and
1 10
} . )•
j .
Lacustrine
····-
-
Figure 26-Rhythmic laminae (varves?) from the central part of Great Salt Lake, Utah, in about 3m water depth. Scale is in millimeters. The lamination is suspected to be due to seasonal alterna tion of organic (a lg a l) production and carbonate ( mostly calcite) precipitation. These laminae formed in a more brackish stage of the lake prior to 12,000 years B.P. The sediments contain a few diatom frustules. ostracodes, and ash layers. Photograph by Kerry Kelts.
Figure 28-Varve laminae from the mid· Holocene in the deep-basin plain of Lake Zug, Switzerl and. Light layers consist mostly of seasonally precipitated calcite; dark layers consist mostly of clay, organic detritus, and diatom frustules. Thin graded turbidites (t) of clastic car bonate episodically interrupt the varved sequence. Scale is in mil limeters. Photograph by Kerry Kelts. Fi gure 29-Varve l aminae from the Rita Blanca Lake Beds of Anderson and Kirkland (1969; early Pleistocene), western panhandle of Texas. Light l ayers consist mostly of calcite, silt-size quartz grains, clay, and ostracodes; dark layers consist mostly of clay and organic mat· ter (Anderson and Kirkland, 1969). Bar.= 0.5 em . Photograph by Douglas W. Kirkland.
Figure 27- Rhythmic laminae (varves?) from Great Salt L a ke , Utah. Bar = 1 em. The thin reddish laminae consist mostly of filaments of red-brown algae. The light layers consist mostly of aragonite and clay. Photograph by Kerry Kelts.
carbonate precipitation in the open lake are illustrated by the profiles in Figure 3 . Because of net photosyn thesis in the epilimnion and net respiration in the hypolimnion, dissolved oxygen is usually saturated or supersaturated in the epilimnion during the day in summer and decreases markedly in the hypolim nion, commonly reaching zero (anox ic). A profile of dissolved C02 would essentially be a mirror image of the 02 profile. Nutrients (illustrated by phosphate in Fig. 3) are depleted in the epilimnion by the phytoplankton
and released in the hypolimnion on decay of the phytoplankton. Removal of C02 by photosynthesis in the epilimnion may increase the pH to 9.0 or more. As a result, the C03 2. to-HC03 ratio increases and the epilimnion becomes supersaturated with respect to CaC03 . Precipitation of CaC0 3 is indicated by depletion of alkalinity (and Ca + 2) in the epilim nion. Some, or in extreme examples, most of the CaC03 precipitated in the epilimnion may never reach the bottom because of dissolution during
Lacustrine
111
Figures 30, 3 1 -Laminated I imestone from the Green River Formation (Eocene), Fossil Syncline, west-central Wyoming. The large clasts are fish coprolites. Photographs by Hans P. Eugster.
settling through undersaturated hypolimnetic waters . This dissolution is less than might be expected because organic coatings protect the CaC03 and reduce the rate of dissolution . Dissolution of carbonate is indicated by a decrease in pH to 7.0 or less and an increase in alkalinity (and Ca + 2) in the h ypolimnion (Fig. 3). As a result of inorganic and/or bio-induced precipitation in open epilimnetic waters, CaC03 in profun-
dal sediments is often predominantly autochthonous . The evidence suggests that most of this autochthonous car bonate is precipitated by phytoplankton photosynthesis (Megard, 1 967, 1 968; Otsuki and Wetzel, 1 974). The greatest effects of diagenesis are dissolution of carbonate due to lowering of pH by oxidation of organic matter in the sediments and the conversion of high-Mg calcite to
Figure 32-Photomicrograph of well laminated, kerogenous, dolomitic marlstone (oil shale) typical of much of the open lacustrine facies of the Green River Formation, Piceance Creek Basin, Colorado. Light layers are composed mostly of altered tuffaceous material and dolomite; dark layers are rich in organic matter. Several groups of con· tinuous laminae are interrupted by boudinage-like pu ll-apart structures call ed loop-bedding. Bar 2 mm. =
112
Lacustrine
Fig u re 33-Photomicrographs showing corre l ation of tuff-rich laminae between two cores from the upper part of the Parachute Creek Member of the Green Rive r Formation, Piceance Creek Basin, Colorado. Distance between core loca tions is about 10 km. Bar = 1 em.
Fig u re 34-0pen lacustrine Pleistocene upper member of the Lisan Format ion, north end of the Dead Sea, Israel. Laminated aragonite (light) and calcite marl (da rk) formed in Lake Lisan, the Pleistocene precursor of the Dead Sea. Begin and others ( 1 974) incl uded these strata i n t h e i r interfan facies. Sedimentary features include minor scour struc tures, thinning and thickening of laminae, and contorted bedding . Rare m i cro-cross laminations, ripple marks, and mud cracks, and common large scale convolute bedding, are present in some beds of this facies (Beg i n et a l , 1 974). Photograph by Peter Scholle.
dolomite. There appears to be a delicate balance between the rate of carbonate precipitation by phytoplankton productivity and the complete destruction of carbonate in lakes that are highly productive . Dean and Gorham ( 1 976) found that lakes in central Minnesota containing sedimentary organic matter in excess of about 390Jo contain no carbonate
in profunda! sediments, whereas lakes in the same area with similar water chemistry but lower organic contents contain abundant carbonate . Decom position of organic matter in the hypolimnion and profunda! sediments apparently generates enough C02 to dissolve any carbonate that may have formed in the epilimnion. Megard (1968) concluded that much of the
CaC03 that is generated by phytoplankton photosynthesis in the epilimnion during the summer and survives dissolution in the under saturated hypolimnion is dissolved in the sediments the following winter. Most of the sediment that ac cumulates in the profunda! zone con tains components that vary in abun dance throughout the year in response
Lacustrine
Figure 36-Cross section of Green Lake, Fayettevi l le, N ew York, showing rel a t ionships between carbonate facies, bedrock types, and bounda ry (chemocline} between the m ixolimni o n and anoxic monimolim n ion in this meromictic lake. The cause of meromixis i n Green Lake is the influx of to seasonal variations in rates of sup- sp r i ngs bel ow about 1 7 m that are more ply. The annual accumulations of s a l ine th an l a k e s u rface waters bec ause sediment may consist of a simple two-Of d isso lved g ypsum from the Silurian component couplet (for example sum- Ve rnon Sha le . Th e over lying Sy racuse mer CaC03 , winter clay) or complex �ormat i�m is als o of Silu :ian age. Loca sequences consisting of many organic t1on of lme of c ros s sect1on _1s shown m and inorganic components, each of F 1g u re 35 .
Fig u re 35-Green (G) and Round (R) Lakes, Fayettevil l e , New Yo r k. Line of cross section in Figu re 36 is shown by the wh ite l i n e . View is approximately to the nort h .
which may have a seasonal pulse in rate of supply. Usually the profunda! zone i s sufficiently oxic to support a benthic epifauna or infauna that will mix the sediments and destroy the delicate annual layers (varves); as a result, the profunda! sediments may be structureless. However, if the pro fundal zone is sufficiently anoxic to exclude benthic organisms , for exam ple if the lake is meromictic, the highly-seasonal components that ac cumulate on the bottom may result in the preservation of annual sediment laminae (Bradley, 1 929a; McLeroy and Anderson, 1 966; Anderson and Kirkland, 1 969; Ludlam , 1 969; Kelts and Hsu , 1978; Figs. 20 to 3 4) . Varves are a powerful i nt erpretive tool for the paleolimnologist because they provide high-resolution time calibration for determining rates and timi ng of lacustrine pro cesses . In ad dition, the varves can provide precise time correlation between locations within the lake basin. In summary, the bulk of carbonate
that accumulates as profunda! sedi ment in marl lakes is the resultant of the rate at which CaC03 is produced in the epilimnion, and the rate at which it is dissolved in the under saturated hypolimnion. Most of this carbonate is Iow-Mg calcite , but high Mg calcite and dolomite may also form if the Mg-to-Ca ratio is suffi ciently high . Although dolomite is commonly assumed to be a mineral that is associated with high-salinity environments (Eugster and Hardie, 1 978; Eugster and Surdam , 1 97 3), it can form in freshwater environments with a Mg-to-Ca ratio > 1 . 0 . Littoral Carbonate
Littoral sediments in marl lakes
usually contain a higher percentage of carbonate than profunda! sediments. This difference is in part due to a greater rate of carbonate production in the littoral zone and in part due to carbonate dissolution in the hypolim-
1 13
nion and on the lake bottom in the profunda) zone. The rate of inorganic precipitation of CaC03 is greater in the littoral zone because of higher diurnal and seasonal temperatures . The rate of bio-induced carbonate precipitation is also greater in the littoral zone because both algae and rooted aquatic vegetation are removing C02. Bio-induced precipitation of CaC03 is also more obvious in the littoral zone because CaC03 encrusting aquatic vegetation can easily be observed. Most detrital sediment is trapped in the littoral zone. This detrital sediment may dilute littoral carbonate if it consists mostly of siliciclastic material, or increase the carbonate content of the littoral sedi ment if it consists mostly of car bonate debris. Biogenic debris may be a major if not dominant source of carbonate in littoral sediments . Remains of calcareous animals, especially mollusks and ostracodes, are com mon . Another source of biogenic car bonate is commonly from charo phytes, calcareous green algae of the genus Chara (Figs . 1 5 to 1 8) . The im portance of charoph yt es as major contributors of CaC03 in marl lakes has been recognized for many years. Charophytes contain both internal and external CaC03. The external CaC03 is mostly precipitated by
114
Lacustrine
Figu res 37 , 38-Aigal b io he rm s , Green Lake, Fayetteville, New York. The main framework of the bioherms is provided by precipi!aled calcite that fills voids and cements sed i m e nt trapped and bound by blue·green a l ga l filaments. The car
bonate cement and sediment are both composed exclusively of l ow-Mg ca lcite.
G rowth of the bioherms begins below the wate r su rface on solid objects such as tree branches (Frg. 39). cans, bottles, and jars (Figs. 40, 4 1 ) . The biohe rms grow upward to the water su rface then outward to form lobate, overhanging ledges that prot r ude 2 to 8 m from the shore (Dean and Eggleston, 1 975; Eg gleston and Dean, i 976).
Figure 39-Beg inning of algal bioherm g rowth on a tree branch from Green Lake, Fayetteville, New York. Scale is in centimeters.
photosynthetic removal of C02 and HC03 in the same way that the other rooted plants in hard-water lakes are commonly encrusted with CaC03 . Internal calcification within the algal cells may amount to more than half of the dry weight of the plant . Most of the internal and exter nal CaC03 derived from Chara can not be recognized as biological re mains, although the calcareous cor-
tication tubules are sometimes preserved in low-energy en vironments. In extreme situations, the stem-like charophyte thalli may serve as nucleae for additional inorganic or organic carbonate accumulation (Dean and Eggleston, in press) . The protective outer cover of the female reproductive structure (oogonium) of Chara calcifies and provides a distinc tive ornamented egg-shaped fossil
Lacustrine
115
Figure 40-Aigal bioherm s tart i n g to g row on tree branches and a zip-top beer c a n in Green Lake, Fayetteville, New York. D ive r is pointing to b ee r can encrusted with low-Mg calcite.
Fig u re 41 -Encrustations of algal precipitated low-Mg ca l c ite (inc ipie nt algal bioherms) on can , bottle, and jar fro m Green Lake, Fayetteville, New York.
the sediments of both modern and
Pyramid Lake, Nevada; but also oc cur in some less saline marl lakes in
ancient lakes.
glaciated parts of the United States.
(Figs. 17, 1 8) that can be common in Algal carbonates (stromatolites) are
common in alkaline saline lakes of
Algal bioherms are particularly well
developed in Green and Round
the western United States such as
Lakes, Fayetteville, New York (Figs.
Great Salt Lake, Utah; Mono Lake,
35 to 42; Brunskill and Ludlam, 1969; Dean and Eggleston, 1975; Eggleston and Dean, 1976). Algal bioherms are
California (Fig. 54); Lake Win nemuc ca, Nevada (Figs, 52, 53); and
Fig ure 42-Charophytes g rowing on the slopes of an algal bioherm, Green Lake, Fayetteville, New York. These calcareous algae are the dominant sedi· ment producers in the littoral zone of Green Lake.
116
Lacustrine
Fig u re 43-Aigal bioherms, Eocene Wilkins Peak Member, Green River For mation, Sweetwater County, Wyom ing (Bradley, 1 929b).
Fi g u re 44-Aigal biohe r ms exposed in plan view on floatblock in the Laney Member of the Gre e n River Formation (Eoc en e ) , Delaney R i m , Sweetwater Cou n ty, Wyoming. Photograph by Emmett Evanoff.
also common in some ancient lake systems such as the Green River For mation (Bradley , 1929b; Surdam and
not particularly common in marl lakes, they commonly constitute a major part of the littoral lithofacies
freshwater lakes. They do occur, however, in streams, in some saline lakes such as Great Salt Lake, Utah
Wray, 1 976; Figs. 43 , 44, 48 to 5 1 ) and the Ridge Basin Group (Pliocene)
in those lakes where they do occur. Oolites composed of low-Mg calcite
of California (Link and Osborne, 1 978; Fig. 62). Lacustrine oncolites (cryptalgal pisolites) have been described from a number of modern and ancient lakes (Fig. 55 , 56, 58 to 62). Although algal carbonates are
have been reported from the margins of Higgins Lake, Michigan, by Wilkinson and others ( 1 980), but to our knowledge, this is the only reported occurrence of oolites and pisolites (other than oncolites) in
(Fig. 65; Eardley, 1 938, 1 966) , Pyramid Lake, Nevada (Jones, 1 925) and hot-spring-fed pools (Figs. 63 and 64; Risacher and Eugster, 1 979). Oolites and pisolites also occur in the lacustrine Green River Formation (Bradley, 1 929b; Picard and High, 1 972) and other ancient lacustrine
Lacustrine
117
Figu re 45-Large hemispherical stromatolite of Oligocene age in the Con· fusion Rang e , Utah. Recrystallization of the carbonate m i ne rals has formed blad ed c rysta l s that may have destroyed fine lam inations that may have existed. Photograph by R. E. Anderson.
Figure 46-Laminated lacustrine a lga l ca rbonate t h at formed around a wel l rounded lithoclast (now re m oved) . Lacustrine beds are part of an unnam ed Ol igocene u n it in the Conger Range, Utah. Photograph by R. E . Anderson.
strata (Figs . 66 to 68). In summary, a typical littoral marl is usually coarse-grained (coarse silt to sand), contains more than 60% carbonate, has a large component of calcareous plant and animal debris and a large component of carbonate precipitated by algal and macrophyte photosynthesis, and may have a large detrital component . This mass of car bonate sediment commonly builds a
platform or marl bench that pro grades outward from shore with a steep drop-off at the lakeward margin . A good example of lacustrine carbonate facies, including oncolites, resulting from development of an ex tensive marl bench in a shallow marl lake in Michigan, is described by Murphy and Wil kinson ( 1 980). They concluded that the progradational marl-bench sequence comprises the
bulk of temperate lacustrine deposits, although the lake they studied had a small profunda! zone. MODEL FOR COMPARISON
The distribution of the interpreted depositional environments of the western part of Lake Uinta, Utah, as it existed in the early Eocene is shown in Figure 72 (modified from Ryder et
118
Lacustrine
Fig u re 47-Recrystal l ized st romatol itic
limestone of the Pliocene and M iocene H o rse Camp Formation
in
the Grant
Range, Nevada. These beds formed at the shoreline of an alkaline lake in a
graben. The beds grade laterally u p depositional dip into cong lomerate and down depositional d i p into recrysta l l i zed thin-bedded l i meston e .
Fig u re 48-Stromatolitic a l g a l bound sto n e in the E o c e n e Dou g las Creek Membe r of the Green R iver Formation in the Tavaputs Plateau, Uta h . Boundstone ov e r l i e s fluvial an d shallow lacustrine s i l i c i clastic beds and was formed at the fluctuating shore l ine of Lake U i nta i n saline water (Fouch et al, i 976).
purposes of this paper, the facies relations diagrammed in Figure 72 serve as a guide for the illustration and interpretation of many of the physical and biological features found in terrigenous rocks of large ancient lakes and lake complexes of the world. The continental sedimentary rocks of the early Eocene of the Uinta Basin can be divided into three major depositional facies : ( 1 ) alluvial, (2) marginal lacustrine, and (3) open lacustrine. Ryder and others ( 1 976) indicated that each of these facies contains a suite of sedimentary struc tures and biologic constituents that collectively indicate the depositional environments of different stages of Lake Uinta's existence. al, 1 976) . This diagram is most ap propriate for one period of Lake Uin ta's existence in northeastern Utah . Kerogen-rich lake beds of the Green River Formation as typically developed in the Green River Basin of Wyoming and the Piceance Creek Basin of Colorado are also present at several different horizons of the Green River Formation in the Uinta Basin, but represent a significant variation from the depositional condi-
tions that existed throughout most of the history of Lake Uinta in north eastern Utah . Although the grouping of the rocks shown in Figure 72 is not appropriate for all lacustrine rocks, it has served as a practical model for in terpreting both surface and subsur face geochemical, biological, and physical rock data. In addition , it separates the rocks into beds that contain probable source, reservoir, and trap units for hydrocarbons. For
ECONOMIC CONSIDERATIONS
Lacustrine carbonate rocks are known to be intercalated with beds composed of saline minerals such as halite, trona, nahcolite, and dawsonite; some of these salts are the focus of mining activities . However, the lacustrine carbonate beds are perhaps most important as sources of petroleum, because they are common ly found in stratigraphic units that yield oil and gas in many parts of the
Lacustrine
Figure 49-Douglas Creek
119
Member of
the Green River Formation, East T avap u t s Platea u , Utah. Stromatolitic algal boundstone (A) overlain by an
ost racoda! , cu rviplanar, laminated to small-scale cross-laminated, ca l c areo u s sandstone (B). The sandstone is ove r lain by a lateral ly-continuous, sma l l-scale, cross-laminated, ostracoda! , g rain· supported limestone (C) as much as 1 m thick that i!3 in t u r n overlain by stromatolitic carbonate rock. This depogilional complex represents cyclic carbonate shoal deposits ove rlain by ter rigeno u s strata formed i n de l t aic and lower d el t a-p la i n environments. Scale is in inches and centimeters.
Fig u re 50-Stromato l itic
boundstone
from east Tavaputs P l atea u , Utah. This sample iR from near the bou n ds to n e of the Green River Forma tion i l l ustrated in
Figure 49.
world, and they form the principal oil-shale deposits of the Green River Formation in the Rocky Mountains of the western part of the United States. Although lacustrine beds have been historically related to com modities of economic interest, published documentation of the com position and significance of the lacustrine carbonate rocks is generally sparse and of limited scope . However, several significant factors seem to indicate that an understand ing of the sedimentology and geo chemistry of carbonate rocks is par ticularly important to petroleum ex ploration efforts in ancient lacustrine depositional systems. The principal petroleum-bearing lacustrine rocks in the world are sandstones, but much of the oil recovered from these units probably formed in kerogenous carbonate rocks and migrated into siliciclastic reservoirs. However, carbonate lacustrine rocks may prove to be im portant reservoirs in the Campos Basin of Brazil (de Castro and Azam buja, in press) and perhaps in other basins of South America and Africa (Brink, 1 974; Brice and Pardo , 1 980; Chignone and De Andrade, 1 970) . Nonmarine carbonate reservoir units in the Campos Basin were apparently
formed from bivalve shells and ostracode carapaces that were concen trated in shoal and beach deposits in shallow water at the margins of lakes during fresher water phases of their history. Some of these shallow-water deposits are redeposited as turbidites near the basin center. These bioclastic sediments were subsequently lithified to form grain-supported rocks.
Bivalve-bearing, grain-supported car bonate units also occur in some of the ancient lake beds of China, but their relation to oil and gas accumula tions is uncertain . In the United States, such bivalve and ostracode grainstone beds are usually less than 1 m thick and are not known to be ma jor petroleum reservoir rocks. However, some of these beds in
1 20
Lacustrine
Fig u re 5 1 -Algal b ioh e rm s , Green River Formation (Eocene). N i n e- M i le Canyo n . Uta h . Photog raph by .Jo h n H . H a n l ey.
Fig u r e 52-Dense Holoc en e stromatolitic algal columns along the western shore of (dry) Lake Winne muc· c a . Nevada . These st ru ct u re s , which grew to heights of 3 m or more, are one morphology of a w i de spec t r u m o f algal carbonates that develop in such semi a rid lacustrine system s . Photograph by Bruce H. Wil kinson .
lacustrine sequences in the central Rocky Mountains are bitumen bearing on surface exposures . Pisolites, oolites, and/or oncolites formed during more saline or alkaline water stages of the lakes of China, South America, Africa, and the United States. Although the role of these grain types in the development of nonmarine petroleum reservoir beds is poorly documented, oolite and pisolite grainstone locally may form relatively small reservoirs for oil in the subsurface in South America and
the United States. These grain types are locally bitumen-bearing on sur face exposures in the Uinta Basin, Utah. Carbonate minerals in Jake beds seem to have played an important role in the formation of porosity in both carbonate and siliciclastic reser voir units. Much of the porosity in the fractured nonmarine Tertiary sandstone of the Uinta Basin is of secondary origin . Fouch and Pitman ( 1 98 1 ) believe that most of the porosi ty developed from the leaching o f
carbonate minerals, and the principle reservoirs for oil in much of the Green River Formation in the Uinta Basin are in fluvial and lacustrine sandstone units in which carbonate grains and cements have been dissolv ed (also see Fouch , 1 975 ; Fouch , 1 98 1 ) . Secondary porosity may prove to be the principle porosity type found in both siliciclastic and car bonate rocks that formed in non marine aquatic environments that contained carbonate-rich water. In systems containing abundant CaC0 3
Lacustrine
121
Fig ure 53-Porous lithoid algal pin
nacles f rom wes te r n (dry)
Lake Win
nemucca, Nevada, These columns, l ike
those in Fig u re
t>2 ,
developed on
coarse,
nearshore, basaltic g ravels and were not associated with spring em a n a t i o ns on
the lake floor. Photog raph by Bruce H . Wilkinson .
F i g u re 54-Algal p i n nacles, Mono Lake, Californ i a . These l a rge algal carbonate
structures form in response to spring discharge d i rectly into the lake. Photog raph by Bruce H . Wilkinson
in solution , parts of both terrigenous and carbonate sediments deposited in or near lakes may be quickly ce mented with calcite prior to the for mation of petroleum and prior to the extensive compaction of grains by burial . In contrast , those grains that were deposited in alluvial settings well removed from the lake may not con tain water rich in CaC03 and may not be cemented with carbonate minerals prior to extensive compac tion . For this reason, the alluvial sediments may be more compacted than marginal-lacustrine sediments, although their burial histories may be similar. Lacustrine and nonlacustrine rocks with little remaining primary porosity may be the end product of early carbonate-mineral cementation
of sediments formed in or near lakes prior to significant compaction com bined with the compaction of sediments deposited in peripheral alluvial settings away from lakes. However , dissolution of carbonate cements can produce abundant pores in those sediments deposited in or near lakes which underwent early cementation by carbonate minerals. Perhaps the greatest economic significance of carbonate lake beds is as a source of petroleum . Fine grained carbonate units form the so called "oil shales" of the Green River Formation. Tissot and others ( 1 978) indicated that the kerogenous car bonate units form the principle petroleum source rocks in Paleocene and Eocene Green River Formation in
55-Beach composed of size oncol ites, Onondaga Lake, N ew York (Dean and Eg, in press).
1 22
Lacustrine
F i g ure 56- Larger oncolites from about 2-m water de pth in Onond aga Lake,
Syracuse. New York. Smaller oncol i tes in upper ri gh t are rounded, rewo rked oncolites from beach for comparison (Fig. 55). M ost oncolites of all sizes contai n one or more stems of charophytes as nucl eae . Charophytes do n ot grow i n Onondaga Lake today and were prob ably elimi nated by a m arked i ncrease in sal i nity of the lake t hat resulted from the i ntroduction of CaCI2 as a biproduct of soda-ash manufacturing on the lake sh o re about 1 885. This d ata suggests that grow1h of the on colites began at least 1 00 years ago. At present, CaCI2 enters the lake through Nine - M i le Creek, cau si ng a marl delta to form at the mouth of the creek (Fig. 57).
F ig u re 5 7 - M a r l delta at the mouth of Nine-Mile C r eek as it enters Onondaga Lake, Syracuse, New York. Th i s example of massive preci pitation of CaC03 i s cau sed by m i xi ng of CaC I2 industrial waste waters with lake waters that are a l rea dy satu rated or nearly saturated w i th respect to CaC03. Sedimentation rates of CaC03 in On on daga Lake m ay be as high as 5 to 1 0 em p e r year (Dean and Egg leston, in press) .
the large oil fields of the Uinta Basin , Utah . In addition, carbonate units
Paleogene lacustrine carbonate and siliciclastic beds o f the Sheep Pass at
contain abundant lipid-rich organic matter in Cretaceous and Paleogene
the Eagle Springs oil field in Nevada (Claypool et al, 1 979) (Fig . 73).
lake beds in the Great Basin of the States (Fouch et al , 1 979). For
United
example , the nonmarine Sheep Pass Formation is believed to be the prin ciple source of oil recovered from
Oligocene fractured welded tuffs and
ACKNOWLEDGMENTS We are grateful to J. P. Brad bury , J . H . Hanley, M . H . Lin k , a n d B. H . Wilkinson for helpful reviews o f the manuscript and to Louise Reif who cor
rected cessor .
its many drafts on the word pro
Lacustrine
1 23
Figure 58-Cross section of an oncolite developed around a gastropod-shell nucleus from O re Lake, M ichigan. Con centric layering is the result of a n n ual couplets of porous and dense laminae (Jones and W i l k i n son, 1 978}. Bar = 2 em. Photograph by Bruce H. W i l k i nson.
F ig u re 59-Cross section o f a n oncolite developed around a gastropod shell of the genus Goniobasis, Flagstaff Limestone (Paleocene-Eocene). Sevier County. Utah (Weiss, 1 969). Ma rks on .sci:ile a r e i n m i l l i mete r s . Photograph by John H. H a n ley.
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124
Lacustrine
F i g u re 60- M a n ' s hand rests on onco l ite packstone i nterca lated with sandstone in the Fl a g st aff M e mber of the Green River Formation on the Wasatch Plateau, Utah . Some oncol ites formed a round the nonmarine gastropods Goniobasis sp and Viviparus sp. (Fouch et a!, 1 976).
F ig u re 6 1 -0ncolites in outc rop of the Flagstaff Member (Paleocene-Eocene), of the Green River Format ion, near Red Narrow:>, U l<:!I J Cou n ty , U t a h . Ligt1t Area in center = Approx. 5 mrn
(Weiss. 1 969).
Figure 62-0ncolite from the n ea r sh o r e lacust rine facies of the Ridge Basin Group ( P l i o ce n e), Cal ifornia (Link and Osborne. 1 978) On colites are associated with remains of ostracodes, mo l l usks . and p l a nts. Photograph by M a r ti n H. Link.
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Lacustrine
125
Figures o3. 64 · · Holocene pisolms. 1 tc 200 mm in dialll eter, forming in shallow. hot-sprmg-fed stagnant pools of water thi:ll are supersaturated with respect to calcite because oi loss of C02 by aigal photosyn thesis and by deg assing, .A.ndean Alti plano. Pastas Grandes, Bolivia (Risacher and Eugster, 1979). Photographs by Har.s P. Eugster.
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1 26
Lacustrine
Fig ure 65-Bars composed of oolites at the shoreline of the Great Salt Lake, Utah. The ba rs are aligned parallel to the shoreline.
Figure 66- Marginal
lac ustr i n e f ac i es . Dou g la s Creek M ember of the Green R iver Formation, Nine-Mile Canyon, West Tavap u ts Plateau, Utah. Piso l itic and ooliti c ostracoda! grainstone (A) (Fouch et a l , 1 976) is capped by stro m at olitic algal boundstone (B). Thi s carbonate complex was deposited i n shoal waters i n Lake U inta. A channel f or med sandstone (C) ove rl ies and l ocal ly scours the boundstone. The ter rigenous beds represent fluvial de p osi tion on a delta (Ryder et al, 1 976).
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__
Lake: Utah Geol. Soc. Guidebook to Geology of Utah, no. 20, p. 105-120. Eggleston, J . R., and W. E. Dean, 1976, Freshwater stromatolitic bioherms in Green Lake, New York, in M. R.
Walter, ed., Stromatolites: Amsterdam,
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Eugster, H. P . , 1970, Chemistry and ori
gin of the brines of Lake Magadi , Kenya: Spec. P aper No. 3, Mineralog. Soc. America, p . 215-235. __ and L . A . Hardie, 1 975, Sedimen tation in an ancient playa-lake complex; the Wilkins Peak Member of the Green
River Formation of Wyoming: Geol. Soc. America Bull . , v. 86, p. 3 1 9-334. __ and __, 1978, Saline lakes, in A. Lerman, ed . , Lakes-chemistry, geology, physics: New York, Springer Verlag Pub . , p. 237-293. __ and R. C. Surdam, 1973, Deposi tional environment of the Green River Formation of Wyoming; a preliminary report: Geol. Soc . America Bull . , v. 84, p. 1 1 15-1 120. Feduccia, A., 1978, Presbyornis and the evolution of ducks and flamingos: Am . Scientist, v . 6 6 , p . 298-304. Fouch, T. D. , 1975, Lithofacies and re lated hydrocarbon accumulations in Tertiary strata of the western and cen-
tral Uinta Basin , Utah, in D. W . Bolyard, ed . , Symposium on deep dril ling frontiers in the central Rocky Mountains: Rocky Mtn. Assoc. Geols., p. 163- 173 . __, 1 979, Character ·and paleo geographic distribution of Upper Cretaceous (7) and Paleogene non marine sedimentary rocks in east-central Nevada, in J . M. Armentrout , M. R.
Cole, and H . TerBest, eds. , Cenozoic
paleogeography of the western United States, Pacific coast paleogeography symposium 3: Pacific Sec . , SEPM, p .
97- 1 1 1 . 198 1 , Chart showing distribution
__,
of rock types, lithologic groups and
Lacustrine
127
Figure 67-Pisol ite grainstone i n the Green River Formation of the Tavaputs Plateau , Utah. These beds form the up permost pa rt of a lateral l y continuous unit that grades u pward from horizontal ly laminated, kerogenous, carbon a te mudstone to smal l-scale c ross-st ratified g rainstone at the top. Th is shoal cycle formed on a beach at the margin of Lake U i nta. This unit is bed A in Figure 66.
Figure 68-Pisol ites exposed at t he marg i n of a M i oce n e and/or Pl i ocene
lake in the Horse Camp Formation , Grant Range, Nevada .
depositional environments for some lower Tertiary and Upper Cretaceous rocks from outcrops at Willow Creek-Indian Canyon through the sub surface of the Duchesne and Altmont
rocks, Uinta Basin, Utah , in R. C. Epis and R. J. Weimer, eds., Studies in Col orado field geology: Colo. School Mines Prof. Cont . , no. 8, p . 358-385. J. H . Hanley, and R. M . Forester, ·
__
Assoc. Geols . and Utah Geol. Assoc . , p . 305-3 1 2 . __ and J . K. Pitman, 198 1 , Sedi mentologic and mineralogic controls on reservoir characteristics of unconven
1 979, Preliminary correlation of
tional hydrocarbon-bearing Tertiary
parts of the Uinta Basin, Utah: U . S .
Cretaceous and Paleogene lacustrine
rocks, Uinta Basin, Utah: Western Gas
Geol. Survey Oil and Gas Inv. Chart, OC- 8 1 , 2 sheets. et al, 1 976, Field guide to
and related sedimentary and volcanic
Sands Project Status Rept. ,
rocks in parts of the eastern Great
oil fields, southwest to north-central
lacustrine and related nonmarine
Basin of Nevada and Utah, in G. W . Newman and H . D . Goode, eds . , Basin
Oct .-Nov.-Dec., p . 7 . (Avail. as Rept . DOE 1 BC1 1 0003 - 1 8 , from Nat . Tech . Inf. Service, U.S. Dept . of Commerce,
d epositional environments in Tertiary
and Range symposium: Rocky Mtn.
Sp ringfield, Va . , 22 1 6 1 .
__ ,
128
Lacustrine
Figure 69-Mudcracks in dolomitic mudstone i n the playa mudflat facies of the Wilkins Peak Member, Green River Formation, G reen River Bas i n , Wyoming Cracks are filled with ca rbonate and siliciclastic sand {Eugste r and Hardie, 1 975) Photograph by Hans P. Eugste r .
Fig u re 70-Lithoclastic packstone inter calated with laminated kerogenous mudstone (oil shale) that was periodical ly subaerially exposed and desiccated. Cracks are filled with ostracode g rainstone. These beds are part of the Green River Formation in Nine-Mile Can yon , West Tavaputs Plateau, Utah.
Ghignone, J . L , and G. De Andrade, 1 970, General geology and major oil fields of Reconcavo Basin, Brazil: AAPG Mem. 1 4, p . 337-358. Hite, R . J., and J . R. Dyni, 1 967, Poten tial resources of dawsonite and nahcolite in the Piceance Creek Basin , northwest Colorado, in 4th Symposium
on Oil Shale: Colo. School Mines
Quarterly, v. 62, p. 25-38. Hutchinson, G. E . , 1 967, A treatise on limnology, v . 2; introduction to lake biolo gy and the limnoplan kton: New York, John Wiley and Sons, 1 1 1 5 p . Jones, J . C . , 1 925 , The geologic history of Lake Lahontan: Carnegie Inst . Washington Pub. 325 , p. 3-50.
Jones, B. F . , H. P. Eugster, and S. L . Rettig, 1 977, Hydrochemistry of the Lake Magadi Basin , Kenya: Geochim. et Cosmochim. Acta, v . 4 1 , p. 53-72. __ and C. J . Bowser, 1978, The min eralogy and related chemistry of lake sediments, in A. Lerman, ed . , Lakes chemistry, geology, physics: New York, Springer-Verlag Pub . , p. 1 79-23 5 . Jones, F. G . , a n d B. H . Wilkinson, 1 97 8 , Str ucture and growth of lacus trine pisoliths from Recent Michigan marl lakes: Jour. Sed. Petrology, v. 48, p . 1 1 03- 1 1 10. Kelts, K., and K . J . Hsu , 1 978, Fresh water carbonate sedimentation, in A. Lerman , ed. , Lakes-chemistry, geology,
physics: New York, Springer-Verlag Pub . , p . 295-323. Link, M . H . , and R. H. Osborne, 1 978, Lacustrine facies in the Pliocene Ridge Basin Group; Ridge Basin , California, in A. Matter and M . E. Tucker, eds . , Modern and ancient lake sediments : In ternat . Assoc . of Sedimentols . , Spec. Pub. No . 2, p. 1 69- 1 87. Ludlam, S . D., 1 969, Fayetteville Green L ak e , New York; 3. The l ami nated sediments : Limnology and Oceanography, v . 1 4, p. 848-857. __ , 1 974, Fayetteville Green Lake, New York; 6. The role of t urbidity cur rents in lake sedimentation: Limnology and Oceanography, v. 1 9, p. 656-664.
Lacustrine
1 29
Fi gu re 7 1 - Tracks of a shore b i rd (note webbing marks} in the Green River For ma tio n , Utah. Tracks were probably made by the flamingolike wad e r Presbyornis whose fossi l re main s are known from local it i es i n the Wi l k ins Peak Member of the Green R i ve r Format ion of Wyoming, Utah, and Colorado (Feduccia, 1 978) Photograph by Hans P. Eugster
I
;
ALLlfiiiAL - MARGIIAL LACUSTRINE r OPfN lACUSTRINE
.
IIUE!OfLTA�
McLeroy, C. A . , and R . Y. Anderson , 1 966, Laminations of the Oligocene Florissant lake deposits, Colorado: Geol. Soc. America Bull . , v. 77, p . 605-6 1 8 . Megard, R . 0 . , 1 967, Limnology, primary productivity, and carbonate sedimenta tion of Minnesota lakes: Univ. Min nesota, Limnology Research Center , In terim Rept . I, 69 p. __, 1 968, Planktonic photosynthesis and the environment of carbonate deposition in lakes: Univ . Minnesota, Limnology Research Center, Interim
r iiiARGINAL I LACUSTRINE
J
tfllH rtUU Hll
Rept . 2, 47 p . Muller, G . , 1970, High-magnesian calcite and protodolomite in Lake Balaton (Hungary) sediments : Nature, v. 226, p . 749-750. __ G. Irion, and U. Forstner, 1 972, Formation and diagenesis of inorganic Ca-Mg carbonates in the lacustrine en vironment: Naturwissenschaften , v . 59, p. 1 58- 1 64 . __ and F . Wagner, 1 9 7 8 , Holocene carbonate evolution in Lake Balaton (Hungary); a response to climate and impact of man, in A. Matter and M. E.
Figure 72-A diag rammatic block diagram (modified from R yd e r et al, 1 976), i l l ustrating the distribution and i n terpreted de po sit i on a l environments o f open-lacustrine, marginal-lacustrine, and a l l uvial facies of the western part of Lake U inta, Utah, as i t existed during the early Eocene. Diagonally striped blue pattern = grain-su pported ca r bon at e rock; solid blue pattern = mud supported carbonate rock; yellow pat tern = sandstone; claystone units are shown in their natural colors. Some thin silicic lastic turbidites a re present at delta fronts and i n the open-lacustrine facies. M uc h of the open -lacustrine facies is composed of kerogen-rich car bonate u n its. The width of Lake Uinta in the d iag ra m i s about 40 km. Vertical ex agge ration is between 1 5 and 20.
Tucker, eds . , Modern and ancient lake sediments: Internat. Assoc . Sedimen tols . , Spec. Pub. No . 2, p. 57-8 1 . Murphy, D . M . , and B . H . Wilkinson, 1 980, Carbonate deposition and facies distribution in a central Michigan marl lake: Sedimentology, v . 27 , p . Otsuki, A . , and R . G . Wetzel , 1 974, Cal cium and total alkalinity budgets and calcium carbonate precipitation of a small h ard-water lake: Archiv fur Hydrobiologie, v. 73, p. 14-30. Picard, H . D . , and L. R. High, Jr . , 1 972, Criteria for recognizing lacustrine
130
Lacustrine
E
w
r--
Figure 73-Cross section i n Railroad Valley, Nevada, i l l ustrating Tertiary sedimentary and volcanic rocks from the Eagle Springs oil field to the Trap Spring oil field. Oil is p roduced from frac tu red lac us t rine carbonate rocks of the Sheep Pa�� Formation and from frac t u red welded tuff at Eagle Springs. Claypool and others ( 1 9 19) ind icated that the oil at Eagle Springs origi nated from the thermochemical matu ration of organic matter in lacustrine rocks of the Sheep Pass Formation. The Ely Forma tion is of late Paleozoic age.
rocks, in J. K . Rigby and W . K. Hamblin , eds . , Recognition of ancient sedimentary environments: SEPM Spec. Paper No. 1 6, p. 1 08-145. Risacher, F., and H. P . Eugster, 1979, Holocene pisoliths and encrustations associated with spring-fed surface pools, Pasos Grandes , Bolivia: Sedimentology, v. 26, p. 253-270. Robb, W. A . , and J. W. Smith, 1 974, Mineral profile of oil shales in Col orado core hole no. l , Piceance Creek
Basin , Colorado, in D. K. Murray, ed . , Guidebook to the energy resources of the Piceance Creek Basin , Colorado: 25th Field Conf. , Rocky Mtn. Assoc. Geols . , p. 9 1- 1 00. Ryder, R. T., T. D. Fouch, and J. H .
Trap Spring
--j
1----
Eagle Springs
other
Paleozoic Other Pale o z o ic rocks
Elison, 1 976, Early Tertiary sedimenta tion in the western Uinta Basin, Utah : Geol . Soc. America Bull . , v. 87, no . 4, p. 496- 5 1 2. Smith , G. 1 . , 1 979, Subsurface stratig raphy and geochemistry of late Quater nary evaporites, Searles Lake, Califor nia: U . S . Geol. Survey Prof. Paper 1043 , 130 p. Smith , J. W . , and W. A. Robb , 1 966, Ankerite in the Green River Formation's Mahogany Zone: Jour.
Sed . Petrology, v. 36, p. 486-490. Surdam, R. C . , and J. L. Wray, 1976, Lacustrine stromatolites, Eocene Green
River Formation, Wyoming, in M. R . Walter, ed . , Stromatolites: Amsterdam, Elsevier Sci. Pub. , p. 535-54 1 .
Tissot , B . , G. Deroo, and A . Hood, 1 978, Geochemical study of the Uinta Basin; formation of petroleum from the Green River Formation : Geochim. et Cosmochim . Acta, v. 42, p. 1 469- 1485 . Weiss, M . P . , 1 969, Oncolites, paleoecology , and Laramide tectonics , central Utah: AAPG Bull . , v. 53, p . 1 1 05-1 120. Wetzel, R. G., 1 975, Limnology: Phila delphia, W. B. Saunders Co . , 743 p . __ and A . Otsuki, 1974, Allochthon ous organic carbon of a marl lake: Ar chiv fur Hydrobiologie, v. 73, p. 3 1 -56. Wilkinson, B. H . , B. N . Pope, and R . M. Owen, 1980, Nearshore ooid formation in a modern temperate region marl lake: Jour. Geology, v . 88, p. 697-704.
1 32
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arbonate dune sands line the coasts in areas of car bonate sedimentation in many parts of the world. Although they form a common and conspicuous facies in Holocene and Pleistocene carbonate sediments, few eolian limestones are reported from ancient rocks. This is either because dune deposits are scarce in older car bonate rocks, or because few geologists have recognized wind-laid limestone. The latter may be the more likely case, inasmuch as recognition of carbonate eolianite is difficult, particularly in the subsurface, because of its similarity to clastic limestone deposited in high-energy, shallow marine environments . The widespread distribution of wind-deposited strata that consist largely or entirely of calcium car bonate was recognized at the start of this century by J. W. Evans ( 1 900) who presented a comprehensive and fairly detailed description of mechanically formed limestone in Kathiawar, India. Since the report of Evans , and especially in recent years, a number of localized studies of calcareous eolianites have been made in various parts of the world - Ber muda, Bahamas, Yucatan , the Mediterranean coast and elsewhere. In most cases , these studies have serv ed to confirm the original ideas con cerning requisite conditions for development. These are: (1) a warm climate favorable for a source of CaC03 sediment; and (2) on-shore winds required to deliver coastal car bonate sand inland to areas of deposition.
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1 33
Figure 2-Air photo showing Pleistocene dune ridges which form the backbone of Isla Mujeres off the Yucatan Peninsula. Looking southeast down th e trend of the middle Upper Ple istocene eolian ridge. Subaqueous remnant of western ridge (right) is seen as l ig ht stripe on right half of photo. Photo by W. C . Ward.
General Description
Most carbonate eolianites are deposited as coastal dunes adjacent to high-energy beaches in warm climates where abundant calcareous sand ac cumulates. Generally carbonate dune rocks consist of well-sorted, cross stratified clastic limestones composed of sand-size carbonate grains blown inland from the beach. Constituent grains are ooids , pellets, and/or any of a wide variety of skeletal frag ments, depending on what organisms are produced in the adjacent shallow marine waters . In some localities the carbonate sand is mixed with quartz sand. Calcareous dunes do not always re main as coastal fringes, and in some regions advance far inland . Most notable is the Thar Desert of India where the wind has transported large quantities of foraminifers inland as much as 400 km or more. The abun dance of their skeletons or tests in the sand decreases only gradually (Goudie and Sperling, 1 977). In the Wahiba sands of Saudi Arabia, the carbonate fraction in some areas is thought to have been blown for great distances from coastal sabkhas and marine ter-
races . Interior desert winds may also build up carbonate dunes, although these dunes are relatively uncommon. In the Southern Desert of Iraq where the carbonate content is generally less than 30 OJo and "is concentrated in the finest fraction ," the main source of the eolian clastics is considered to be the Mesopotamian Plain (Skocek and Saadallah, 1972). Along the Per sian Gulf in Saudi Arabia and Iran , fine sand and dust, some of it highly calcareous, has been transported long distances from the northwest by the prevailing wind (Emery, 1 956). Other illustrations of interior source areas for carbonate clasts are in the lea Desert of Peru where Newell and Boyd ( 1 955) observed dunes formed largely of Eocene mollusk shell fragments, and in the Great Salt Lake Desert, Utah, where Jones ( 1 953) recorded ooids, gypsum fragments,
ostracod clasts and algal particles, all derived from marginal deposits of a former large lake. Diagnostic Criteria
The depositional environment of calcareous eolianites is notably dif-
ficult to recognize with certainty in many places, as has been pointed out by Glennie ( 1970) and by Ward ( 1 975) . Few examples have been described from ancient rocks, both because of their similarity in composi tion to sands of other depositional environments and because alteration through diagenesis commonly makes recognition very difficult. The prob lem of determining with confidence the origin of these deposits is well il lustrated by the Miliolite Limestone of western India and adjacent regions which has been referred to coastal marine deposition by numerous geologists and to eolian deposition by many others. Likewise, the Gargaresh and related limestones along the Mediterranean shores have been variously attributed to both marine and continental genesis (Hoque, 1975) . Because the clastic constituents of most calcareous eolianites are derived largely from the skeletons of marine organisms such as bivalves, corals and foraminifers, or from inorganic grains such as ooids, they are com monly identical to sand grains on nearby beaches or bars. Composition,
1 34
Eolian
Figure 3-Holocene eolian ite back of beach on Isla Blanca, Yucatan Pe nin· sula. Exposure face is parallel to stran d l i n e . Maximum height of dune is 2 . 5 m . Phot o by W. C. Ward.
therefore, cannot normally be con sidered a reliable means of distinguishing between such en vironments. The presence of marine organisms in eolian deposits doubtless has led to many misinterpretations as is indicated by numerous examples of facies assignments that were based en tirely on the presence of certain specific animals. Where the clasts in eolianites in clude fragments from the shells of terrestrial animals, such as land snails or from the bones of vertebrate animals, they mostly, but not always, represent nonmarine conditions, and when considered with other factors such as wind direction, may support an eolian interpretation. Various features of texture seem to be characteristic of eolian deposits, but most also occur in other en vironments and cannot be considered proof of wind deposition. Among these features are roundness, polish, and abrasion of grains, all of which are likewise common attributes of
beach and some other sands (Folk, 1967). Because such features normally reflect the process of transportation and not of deposition, and may represent a second or third generation of sedimentation, they are not valid evidence of wind activity. Good sort ing and the winnowing out of fine particles (dust) likewise are characteristic features of most calcareous eolianities, but are by no means restricted to sands of that en vironment. A feature common to many eolian limestones and carbonate dunes, but seldom observed in beach or marine deposits, is the preservation of root casts and molds. These are recorded as common in Australian eolianites (Teichert , 1947) and are prominent in the Bahaman deposits (Ball, 1 967), as well as in many other places. Probably the most diagnostic criteria for the recognition of calcareous eolian deposits, as with quartz sand deposits, are the struc tures . Cross-strata constitute the
largest and principal type of struc ture, but unfortunately few geologists have given quantitative descriptions of the dip, length of foresets, dimen sions of sand body, or other signifi cant features. Of ten descriptions of calcareous eolianites recently review ed, nine described their structures as "distinctive cross-strata," "dune-like cross-strata, " "well-developed false bedding" or used other nonquan titative statements. Most of the characteristics of cross stratification attributed to dunes of quartz sand (McKee, ed . , 1 979) seem to apply also to calcareous dunes. Large-scale foresets - dipping sur faces that are greater than 20 ft in length and may be 60 or 70 ft in length - are common features. Degrees of dip of foresets are mostly high angle, commonly 30° and, in some deposits, 33 or 34° . The basal parts of foresets commonly have a tangential relation to the underlying surface -- a result of the large amount of suspension-load deposi-
Eolian
Figure 4-Air photo showing m ultip le ridges (upper left) on Isla Blanca bar r i er island, Yucatan Peninsu la . Dune, beach. and nearshore carbonate sands ha ve thin oolitic coatings. On landward side (left beyond photo) is large lagoon of c arbonate mud. S m a l l isol ated tida l pools (left side of photo) beco m e h yper sal ine i n some se aSO·'�S. Photo by W. C.
135
dt.ne
Ward.
tion. Although most eolian dunes have cross-stratification that is either tabular-planar, or wedge-planar, some coastal dunes of carbonate type develop trough structures with a fes toon pattern as described from the Bahamas (Ball, 1 967) where they fill saddles between lobes. Also common on some calcareous dunes are the convex-upward foreset structures distinctive of many parabolic-type dunes. This structure, common in Bermuda, has been discussed by MacKenzie ( 1964a, 1 964b) who stress ed its unique character in this type of dune.
Some minor structures, though relatively scarce, are among the best criteria for the recognition of eolian deposition in carbonate deposits. Rip ple marks are preserved uncommonly but, where found, constitute excellent evidence of wind activity. They are readily distinguished from water formed ripple marks both by ripple index and by their distinctive orienta tion parallel to the dip of high-angle foresets (McKee, 1945). Raindrop pits formed in dry sand likewise may be readily recognized by their cup-like craters, commonly reoriented on a sloping sand surface as described and illustrated by McKee ( 1 945). Foresets of eolian sands may "ex hibit crumpling of some laminae" as stated by Glennie ( 1 970). Further more, distinctive types and combina tions of contorted beds are characteristic of wind-deposited sand, and serve to differentiate between various degrees of cohesion as in dry, wet, saturated, and crusted sand.
Fig u re 5-Eolian lim estone of Bahama I sland. Ridge of eolian ite A (top), B (bot tom) n e a r Eleut h e ra Island bordering north east Providen ce Cha n n e l . Photographs by Peter A. Scholle.
136
Eolian
F i g u re 6-Cross-strata A (right), B (below) in eolian limestones, Government House Lake, Rottnest Island, Western Australia. Photog raphs by C u rt Teichert, 1 946.
Such structures have been determined by laboratory experiments (McKee, Douglass, and Rittenhouse, 1 97 1 ) and tested on coastal dunes of Brazil by McKee and Bigarella ( 1 972) . Summary of Criteria1
The following summary of the pro perties of Quaternary eolianites may aid in the identification of eolian limestones : (a) geometry - generally elongate bodies of carbonate grainstone tren ding parallel to the strandline; thickness highly variable within a single body; (b) associated facies - interfingers with beach and near-shore sands of similar composition; in some settings interfingers landward with lagoonal mud or evaporite-pond sediments; (in prograding sequence, overlies beach sand and might underlie lagoonal deposits or caliche); (c) composition - sand-sized skeletal fragments, ooids, pellets, and ' Prepared by W . C. Ward .
Eolian 1 37
Fig ure 7-Biock diagrams of: A transvers€. 8 barct1anoid ridge. C bar cha n , D blowout, and E parabolic dunes, (from M c Kee, ed , 1 979).
peloids without larger marine shells such as commonly occur in beach and nearshore carbonate sands; (d) grain size and sorting - fine- to medium-grained without the gravel sized constituents which are common
(g) trace fossils - rhizocretions, root-hair sheaths, and micro-bori ngs associated with root systems of dune plants; burrows rare and; (h) other evidence of subaerial deposition - paleosols ; land snails
cross-strata) and g (lack of burrows) should be added to the combination. Paleosols and terrestrial fossils are diagnostic, but are uncommon . Penecontemporaneous vadose cements, rhizocretions, and caliche
in beach deposits; mostly well-sorted ; (e) stratification - alternating fine and coarse laminae; generally highly cross-stratified on large scale, high angle foresets dipping predominantly
and other terrestrial fossils; subaerial
can develop in marine grainstones
l an dward
(t)
.
early-stage cements - vadose
cements such as meniscus, pendulous, and needle-fiber;
crusts (caliche layers); karst features.
which are deposited
Of these characteristics the most diagno stic for subsurface ev a l uati o n is probably the combination of b
coastlines.
(vertical
seq uence) and d (well-sorted
carbonate grainstone without coarser
material) . Where cores or dipmet er data are available, e (high-angle
along prograding
Recognition of carbonate dune rock in the geologic record is impor tant for several reas on s : ( 1 ) Iden tification of eolian l imeston es facilitates location of ancient strandlines and islands - this is im-
1 38
Eolian
Figure 8-Cala Conta, lbiza Island, Balearic Islands, Mediterranean. A (right), Pleistocene eolian limestone with high-angle c ross-strata and caliche cap; red-brown colluvial silt below; 8 (below}, Holocene eolian dune showing root system; scale in center is 20 em. Photog raphs by C.F. Klappa.
portant in stratigraphic analyses and in studies of early diagenesis; (2) Eolian ridges have profound effect on
cemented early, but not always per vasively, enhancing the chance that some primary and early-secondary
the distribution patterns of subse
porosity may be preserved after com
quent carbonate sediments. Recogni
paction during deep burial. In some carbonate sequences, eolianite is the most-updip facies and may interfinger with lagoonal-mud source rocks.
tion of dune-ridge limestone in the
rock record, therefore, should be significant in predicting facies
distribution in overlying limestones; and (3) Eolianites are potential oil and gas reservoirs because they are
Terminology The sediments and their lithified equivalents that are treated in this chapter have two distinctive features - a composition that is dominantly calcium or magnesium carbonate and a genesis through eolian processes. Thus, it might be supposed that a name encompassing these two elements might readily be determined and receive general accep tance. A survey of the pertinent literature, however, shows that no one term has had exclusive adoption and that approximately 20 names or variants (Table 1) have been applied to the calcareous dune deposits and consolidated rocks derived from them . As indicated in Table 1 , the feature of CaC03 composition has been variously indicated in the sediment name or rock name by use of such terms as calcareous, carbonate, c a lcarenite , lime or limestone, or when the grains consist of a single distinctive type, by such terms as
oolitic or foraminiferal. The agent of deposition is mostly indicated by the names eolian , wind-blown , dune sand or dune rock. In many descriptions, one or the other of these elements is
Eolian
139
TABLE 1 Locality
Name Calcareous dune rock Calcareous dune formations Calcareous low dunes Highly calcareous and cemented dune sands Wind-laid calcarenite Wind-formed calcareous rock Wind-blown lime sand Carbonate dunes Carbonate dune rock Carbonate-sand eolian dunes Wind drift shell sand Wind-blown calcareous deposits Dune lime sand Dune limestone Aeolian calcareous rock Eolian (aeolian) limestone Eolian carbonate sand Eolian ridges, carbonate sand Eolian calcarenites Cross-stratified oolitic rocks, fossil dunes Foraminiferal aeolian deposits Gypsum-oolite dune complex Gypsum-oolite dunes
Reference
Australia Australia India Thar Desert, India
Fairbridge (1950) Bird (1972) Evans (1 900) Goudie and Sperling (1977)
Yucatan India Bermuda Bahamas; libya, Saudi Arabia Yucatan Yucatan Bermuda India Onotoa AtoH Australia; · India India Bermuda; Yucatan Australia; Bermuda Bahamas Bahamas Australia Tunisia
Ward and Brady (1973) Evans (1900) MacKenzie (1964) Ball (1967); Glennie (1 970) Ward (1 976) Ward and Brady (1973) Verrill (1907) Evans (1900) Cloud (1952) Teichert (1 947); Shrivastave (1 968) Evans (1900) Sayles (1931 ); Ward (1 975) Teichert (1947); Verrill (1907) Ball (1 967) Ball (1967) Bird (1 972) Fabricius (1970)
lndta Utah Utah
Goudie and Sperling (1977) Jones (1953) Jones (1953)
Table 1 . Terms applied to wind oeposited dunes, dominantly of CaC03 g ra i 'I S . a nd the1r l 1 thified equivalents.
implied but not specifically stated in discussio n s , so t h at such t erms as dune sand. calcareous sand, o r dune rock must be in context to be clearly understood.
particular rock unit should he called a
of t he principal coastal localities
calcareous eolianite or an eolian
t h roughout the world where car
limestone (stressing the process or t he
composi tion ) , however, seem� matter of desired emphasis.
stabili zed with t i m e . there seems t o be great variation in the amount of t i me
Geography Carbonate dunes and eolian limestones probably h ave a more world-wide distri bution than is generally recognized (Fig. I ) . How eve r , they an: largely rest ricted to regions of warm climate and they mostly form adjacent to coast al areas . They are uncommon in interior deserts, but may develop locally where alluvial fans furnish a source of carbonate sediment or where calcareous part i cles of an earlier generat ion have accum ulated . A list
required for li t h i fication , so only in a
cemented sands a Pleistocene or earlier age. The name "eolianite" was propos
ed by Sayles in 1 929 " for all con solidated sedimentary rocks deposited by the wind" ( Sayles , 1 93 1 ) ; where they were composed l argely o f calci um material . t hey were termed
"calcareous eolianite . " Whether any
present ed i n Table
2.
relat ively unimportant and is mostly a
Although modern dunes are mos t l y active, a n d become cemented and
very general way can active dune sands be assigned a Holocene age and
bonate eolianites are recorded is
DEPOSITIONAL SETTING Location and Development of Dunes Typi cal calcareous eolianites may be mechanical deposits of pure CaC03 or may be m ixed i n various proport ions with quartz sand and other detrital grains . They characteristically h ave a simple t ex t ure , being formed of accumulat i ons of fossil fragments (bioclast s ) . ooids , pellet oid s , or microfossils s u ch a� forami n i fers, or t h ey may contain a mixture of t hese . Most carbonate dunes have a basic structure con sisting o f cross - st rata of t wo pri ncipal
1 40
Eolian
TABLE 2 References
Locality Bermuda Bahama Islands Yucatan Coast
Verrill, 1 907; Sayles, 1931 ; MacKenzie, 1 964a, b.
�y, 1 947; llling, . Ward and Bra� 1973; Ward and Newell and R 1 954; Ball, 19
Wilson, 1974; arms, Choquette and Brady, 1974; Ward, 1 975.
Baja California
Ande�n. 1950; Ph�r and Ewing, 1 962.
Northwestern India
Chapman, 1900 ; Glennie, 1970; Lele, 1973.
Persian Gulf
Evans, 1 900.
Tunisia
Fabricus, 1970.
- ��---
-·�---�-�
Glennie, 1970; Hoque, 1 975.
Egypt
Selim, 1974.
Israel
Yaalon, 1967.
Greece; Malta
Hoque, 1975. Hoque, 1975.
West Australia
types - low-angle windward slopes and high-angle foresets. As with quartz sand dunes , the upwind strata arc largely destroyed during dune ad vance and only the high-angle cross strata of the lee side normally are preserved. Carbonate eolianites, in general, are greatly affected by diagenetic changes, especially cemen tation, early in their development. The initial stages in dune-forming along a coastal area consist of the transfer of dry beach sand or bar sand by onshore winds to form one
-
Libyan coast
Algeria; Italy
Table 2. List of principa l coastal localities contaming ca rbonate dunes and/or eolian l imestones.
�-- ·
Teichert, 1947; Fairbridge, 1950; Malek Aslani, 1 973.
Victoria, Australia
Bird, 1972.
South Australia
Von Der Borch, 1976.
Onotoa Atoll, Gilbert Islands
Cloud, 1952 .
Christmas Islands
Fosberg, 1 953.
Galway, Ireland
Evans, 1 900.
Cape Town, South Africa
Evans, 1 900.
·
---
Great Salt Lake Desert
Jones, 1953.
Thar Desert, India
Gaudie and Sperling, 1977.
Southern Desert, Iraq
Skocek and Saadallah, 1 972.
lea Desert, Peru
Newell and Boyd, 1955.
Oman
Glennie, 1970.
or more ridges parallel to the coast . In Yucatan (Figs. 2 , 3 , 4) such ridges border the Caribbean Sea and com monly separate lagoons from the sea (Ward and Brady, 1 97 3 ), whereas in the Bahamas (Fig. 5A, B) long dune ridges form low islands ( llling, 1 954). Along the southern ocean coast of Victoria, Australia, " prevailing westerly winds are driving sand inland and crests rise more than 200 feet above sealevel" ( Bird , 1 972). In western AusLralia (Fig. 6A, B) numerous parallel rows o f beach
ridges show "an element of dune ridge about them" (Fairbridge , 1 950). The multiple seaward ridges of the Bahamas (Ball, 1 967) are said to com· pose the highlands of most islands . These ridges parallel the slope breaks, especially the beaches and bars . Largest dunes are upwind, on the eastern edge of the platform which is near the source, and the smallest dunes are downwind (Rail, 1 967) . In Baja California, dune fields trend parallel to the barriers along the coast ( Phleger and Ewing, 1 962) .
Eolian
141
Fig u re 9-Punta Chincho, lbiza Island, Balearic Islands, Mediterranean. A {left), Ple istocene carbonate eolianite; upper part c ross-st ratified, lower part contains calcified root system; 20 em scale. B (below), H igh-angle (30°) cross-strata in calc areous dune sand; nodular ap pearance from calcified roots and salt weathering; 20 em scale. Photog raphs by C. F. Klappa.
Along the coast of western India, ridges of calcareous eolianite are common on sand bars, marking the present coast line and standing about 1 0 ft above sealevel (Verstappen, 1 966). Earlier dune ridges are record ed to have been formed on old shorelines by the advancing Pleistocene sea (Shrivastava, 1 968). On atolls of the Pacific, carbonate dunes are formed on the lagoon side of gravel ramparts where sand, large ly of foraminifers, is blown from the adjoining reef flat (Cloud, 1 952) . Figure 3 of Cloud's report shows dune locations with respect to the reef flat. Most atoll dunes are not more than 2 to 3 m above high tide accor ding to Fosberg ( 1 953). Following the development of many calcareous dune ridges, the pro cesses of cementation and stabiliza tion begin almost at once, so dunes of early Holocene and Pleistocene age are commonly lithified. Most calcareous eolianites clearly indicate by the directional vectors of their cross strata that they were form ed by prevailing winds or by at least reasonably constant on-shore winds. This feature accounts for the normal transverse orientation of dune ridges and for the common development of
such dune types as the transverse, parabolic, and blowout. On the Lib yan coast , however, detailed measurements by Hoque ( 1 975) show ed evidence of bidirectional winds with azimuthal modes of northeast and southeast. Causes or controls of the alter-
nating sequences of calcareous eolianites and soil zones in Bermuda, India, Yucatan and some other areas have been considered by various geologists (Sayles, 1 93 1 ) to be changes in sea level during the Pleistocene. Periodic drops in sea level, attributed to the forming of ice
1 42
Eolian
Figure 1 a-Ple istocene eol ian ite. I sla Mujeres off the Yucatan Peninsu l a . Up per layers of weakly cemented eolianite with thicket of rhizocretions weathering in re l i ef . St ructu reless layer (0.3 m thick) is paleosol which contains land snails and calcified cocoons of soil-dwelling weevil. Fewe r rhizocretions penetrate lower unit. Photo by W. C. Ward.
sheets, are considered favorable to ex posure of much sand for dune build up and so, at least in Bermuda, a glacial-interglacial stratigraphic se quence has been recognized. Dune Types
The classification of dune types that is used in this discussion of eolian limestones is that recommend ed in U . S . Geological Survey Profes sional Paper 1052 on Global Sand Seas (McKee, ed . , 1 979) . This classification segregates into types the various dune forms and relates these types to specific environments of deposition . This classification is based on two principal attributes of dunes - the shape or form of the sand body and the position and number of slipfaces (steep lee-side surfaces) on it. Although the recommended classification was developed largely from studies of quartz sand dunes of interior deserts, few if any differences between the largely quartz and largely carbonate dunes can be recognized in either the operative processes or in the resulting dune forms . Mostly the di fferences are in the proportions of each dune type represented and, of course, in their composition .
Fig u re 1 1 -Caliche crust lining soil
pocket formed as solution hole in Pleistocene carbonate dune roc k .
E leuthera I s land, Bahamas. Ph oto b y E . A Shinn.
Because nearly all extensive deposits of eolian limestones are formed on subaerial surfaces border ing the sea in warm temperate latitudes or in arid climates, most of the dunes of which these deposits consist are the same types as those commonly recognized in coastal areas of quartz sand accumulation. Most of
the dunes result from redeposition of clasts by unidirectional onshore winds that transport carbonate sand com posed of skeletal fragments, foraminifers, ooids or other types, from nearshore marine sources , especially beaches. The transverse dune (Fig. 7A) is the most commonly mentioned dune type in the literature on carbonate eolianites. The reason being that most relatively straight dune ridges, parallel to the coast , are normally referred to as transverse dunes. Such ridges with foresets dipping shoreward are a com mon feature in the Bahamas (Ball, 1 967) and are found on the Yucatan coast , bordering beaches (Ward, 1 975) . Along the coast of Libya (Ho que, 1 975), Pleistocene dunes developed where "carbonate beach materials were carried landward to form transverse sand dunes . " The relationship of transverse to bar chanoid forms (Fig. 7 A, B, C) is il lustrated in Bermuda where transverse types have been described (MacKen zie, 1 964b) as having developed from the coalescing of small dunes to form ridges. In Great Salt Lake Desert , large lateral carbonate dunes are referred to (Jones, 1 95 3 ) as a transverse type "bordering on the
Eolian
143
Figure 1 2-Pieistocene e olianit e on Isla Mujeres, Yucatan Peninsula, Mexico. A
(left), Abundant rhizocretions; grains skeletal. B (below), Dominantly skeletal grains. Photographs by M. Esteban.
barchane shape . " Barchan dunes (Fig . 7C), which are probably the most elemental type, are characteristically formed in areas of unidirectional wind and relatively small sand supply. In eolian sands of the Southern Desert in Iraq , they ap parently are fairly numerous and oc cur in many places, but the carbonate content in this sand is relatively small , "reaching a maximum of 30 percent" (Skocek and Saakallah , 1 972). These dunes are not coastal. On a barrier bordering tidal lagoons of Baja California, extensive barchan dunes, "many more than 100 feet high ," are reported by Phleger and Ewing ( 1 962) , but here also the car bonate content of the sand is very low - in this area only about 1 o/o foraminifera and shelf - so they can not be classed as eolian carbonates. Other examples indicate that the bar chan dune is not typical of carbonate eolianites. Distinctive Dune Types Certain dune types in the recom mended classification form a distinc tive evolutionary sequence under con ditions of unidirectional wind, with a sand supply that is decreasing. This charactertistic sequence involves change from transverse to barchanoid ridges to barchan dunes. Such a se quence is typical of many interior deserts where vegetation is sparse (McKee, 1966) . The tendency toward
development of a similar sequence, with parabolic dunes instead of bar chan dunes and with rows of coalesc ing parabolics rather than barchanoid ridges, occurs in areas of considerable vegetation, including some coastal areas containing calcareous eolianites. Thus, in regions of partial dune stabilization, compound parabolic dunes that consist of U-shaped or V-shaped sand hills combined in rows to form ridges, seem to be the counterparts of desert barchanoid ridges . Rows of compound parabolic dunes seem to be especially well
developed in the Thar Desert of India where they have been referred to (McKee, ed . , 1979) as "rakelike" in form because of the resemblance be tween the nearly parallel trailing arms and the tines of a rake. The amount of carbonate sand in these dunes is not recorded, but their extensive development in northwestern India probably reflects the control of sand movement in that area by monsoon rains and by resulting vegetation. In the western Rajasthan desert near Bikaner such dunes are described as "well stabilized" and include "layers of calcium carbonate" (Saxena and
1 44
Eolian
Figure 1 3A,B-Pre-Holocene and Holocene eolianites at Canc u n , Yucatan Peninsula , Mexico. Most grains are marine ooids. Photographs by M. Esteban .
Singh, 1 976). They are said to be mostly coalesced and to "run in a chain for a distance of 1 to 2 km . " The term "coalesced parabolic" dune is used by Singh ( 1 977) for this dune type. In the coastal areas of the Bahamas, lobes that seem to be com parable to the noses of parabolic dunes and are distributed along calcareous sand ridges, are described by Ball ( 1 967) and referred to as "eolian spillover lobes." As il-
lustrated by Ball (1967) , some eolian ridges that form "the multiple seaward ridges . . . on most of the islands' ' are largely composed of these lobes. They have been partially stabilized by vegetation as indicated by root casts, and the cross-strata dips "range through an arc of about 1 80 degrees centered onto the plat form , " both of which features in dicate their affinity to the parabolic dune type. In Bermuda, as in the Bahamas ,
dune ridges formed from the combin ing of lobes seem to be characteristic . As described by MacKenzie ( 1 964a) "the individual dunes appear to be predominantly lobate sand bodies which have coalesced to form ir regularly defined transverse dune ridges . " In these deposits the wind ward side is "markedly irregular; fes tooning, scour-and-fill and filled blowout surfaces are common" (MacKenzie, 1 964b). These ridges, therefore, represent a form of com pound parabolic dune. Blowouts (Fig. 7D) and parabolic dunes (Fig. 7E) , as might be ex pected, constitute common forms in some coastal fields of carbonate sand, especially where vegetation is abun dant and serves to partly stabilize the sand. In Bermuda, lobate sand units that have convex upward foresets and curved crest lines that are concave up wind are interpreted as "trace parabolic or U-shaped dunes" (MacKenzie, 1 964a). In the Thar Desert of India, where dunes "consist mainly of clustered parabolic and obstacle" types, sands are described as "highly calcareous and cemented dune sands" and as " foraminiferal aeolian deposits" (Goudie and Sperl ing, 1 977). Interdunes Because a large percentage of car bonate dunes seem to have formed as
Eolian
1 45
Fig u re 1 4-Wedge-planar c ross-strata o f mixed q u a r t z and carbonate s a n d ; low angle strata ove rlying h ig h a n g l e ; Per sian G u lf coastal d u n e . Photo by E . A. Sh i n n .
Figure 1 5-Stratified carbonate eolianite, Rottnest Island, Western A ustralia. A (right), Phillip Point; foreset beds dip below sea level. B (below), Lake Bagdad. Photographs by Curt Tei chert, 1 938.
"beach ridges ," the nature of inter dune corridors between them should be predictable. The process of form ing interdunes consists of the ac cumulation of fine sediments settling from suspension or introduced by waves or tides, together with unsorted debris and, locally, with the car bonaceous remains of plants. The ex tent and ultimate preservation of
these deposits is largely dependent on the rate of dune migration across the interdune surface as described in McKee and Moiola ( 1 975). Unfortunately, few records of the textures and structures o f interdunes between calcareous eolianites have been made. In Yucatan , where exten sive dune deposits occur, interdune structures are described by Ward
( 1 975) as "near-horizontal strata . " In the Great Salt Lake Desert , Utah, carbonate eolian cross-strata (Jones, 1 953) are said to be overlain by near ly horizontal layers of windblown material. These structures are expec table for interdunes . In some areas, however , similar characteristics are ponded surfaces or ephemeral lakes where muddy sediment accumulated and tidal flats developed . Interdune deposits along the Libyan Coast are described (Hoque, 1 975) as red-brown units with high clay and silt content , but no cross-strata. The thickness of an interdune deposit is clearly a function of the time available for accumulation before burial by an encroaching dune. Thus, where dunes tend to stabilize early because of cementation or other diagenetic processes, relatively large amounts of fine sediment may be deposited, and weathering processes may act upon the sediment to develop a soil. In Bermuda, where a series of five soils is recognized between dune deposits, such soils have been inter preted as the products of interglacial times when sea level was high (Sayles, 1 93 1 ) . Paleosols likewise are recogniz ed within dune sequences in coastal
146
Eolian
F;gure 1 6A,B-Pieistocene eolia nites o:1
M a llorca. Balearic I slands. Abundanl rh izocretions; fragments of mollu sks, rea a lgae. bryozoans, ech inoids. Tabular planar c ross·strat a . Photographs by M . Esteban .
primary or penecontemporaneous product formed "beneath interdune flats between stranded barriers" in the Coorong area of South Australia (Von Der Barch, 1 976). The dolomite is associated with structural elements characteristic of shallow-water to subaerial origin such as desiccation cracks, intraclast breccias and bird's-eye fenestrae . Height of Calcareous Dunes and Thickness of Eolian Limestones
Libya (Hoque, 1 975), and "interbed ded with dune sands" in southeastern Australia, as well as a few other areas. Among the low dunes of Onotoa Atoll in the Gilbert Islands, what might be considered a dune-interdune sequence is described by Cloud ( 1 952) as similar to modern dune sands "but with a humus layer weakly to moder ately well developed . " Although sabkahs, ephemeral lakes and other water bodies - both tidal and fresh - may develop in inter-
dune areas, not all such basins of deposition can be considered as inter donal . They may merely represent features of an environment that hap pened to be in the vicinity of eolian accumulation, but are developed quite independently of the eolian processes . The result of flooding between dune ridges by marine currents and the complicated distribution patterns in delta-like fashion is described for the Bahamas by Ball ( 1 967). Microcrystalline dolomite (less than 10 to 20 microns), is described as a
Few reliable measurements are available on the size of modern calcareous dunes. In many places, such dunes occur on the tops of con solidated dune rock (Table 3), most of which is probably of Pleistocene age , but the older dunes are not always distinguishable from nonlithified or partly lithified Holocene dunes. On the basis of the sparse data, calcareous coastal dunes must be considered relatively small , as compared to many large quartz sand dunes of interior deserts; com monly they are less than 100 ft high (Table 4) . Pleistocene eolian limestones which occur primarily in coastal areas of various parts of the world have been the subject of many studies, but , as a rule, these limestones have not been dated with respect to the time span represented, or even in many places ,
Eolian
147
TABLE 3 1 . Isla Mujeres, Yucatan (Ward, 1 975, p. 51 1 ). Younger [Pleistocene] dunes blown on top of earlier dunes. 2. Isla Cancun, Yucatan (Ward and Brady, 1 973, p. 233). Active dunes piled against and atop of Pleistocene dune rocks. 3. Victoria, southeast Australia (Bird, 1 972, p. 349). Modem dunes are on older lithified dunes. 4. Point Peron, western Australia (Fairbridge, 1 950, p. 39). Holocene dunes overlie older consolidated dune rocks. 5. Abrolhos Islands, western Australia (Teichert, 1 947, p. 1 53). Partially cemented dunes overlie Pleistocene dunes. 6. Coastal terrace, southern Tunisia (Fabricius. 1 970, p. 757) "A rim near present-day shoreline is of Holocene age. · 7 . Libyan coast west o f Tripoli (Glennie, 1 970, p . 1 28). Uncemented Holocene dunes overlie lithified Pleistocene dunes.
Table 3 . Relationships of some calcareous dunes to carbonate dune rocks.
Fi g u re 1 7 -H olocene eolianite on Isla Cancun, Yucatan Pen insula. Ex posure face paral lel to dip of leeside foreset lami nae (landward is to the right). Photo by W. C Ward.
the part of the Pleistocene in which they were formed . Figures for the thickness of principal eolianite limestone formations general ly range from 1 00 to 200 ft (Table 5), although in some areas, such as Ber muda, the maximum thickness is con siderably greater (Verrill, 1 907). Here, the total t hickness apparently represents the sum of several deposits of wind-accumulated s an d each con,
trolled by changing base l evel , and separated by soils or other products of nondeposition . Paleosols and Stabilization by Plants
Carbonate dune sands are stabilized not only by partial or complete cementation of particles, but also by vegetation , especially where roots are extensive (Fig. 8B). Root casts rhizocretions of Kindle ( 1 923) or
dikaka of Glennie and Evamy ( 1 968) - where well preserved, give evidence of partial stabilization in some places as in Bermuda (Verrill, 1 907), on the Bahama Banks (Ball, 1 967), and in Victoria, Australia ( Bird , 1 972). Calcified root systems are well illustrated i n the Balearic Islands of the western Mediterranean (Figs . 9A, B) . Paleosols occur as layers within se-
148
Eolian
TABLE 4 Locality
Height (In feet)
Reference
88 1 00 +
Fairbridge (1 950, p. 39). Phleger and Ewing
Isla Blanca, Yucatan
12
Ward and Brady (1 973,
Isla Cancun, Yucatan
50
Ward ( 1 98 1 , pers. commun.) Verrill ( 1 907, p. 49).
Western Australia Baja California
Bermuda Joe's Hill, Christmas Island Great Salt lake Desert, Utah
( 1 962, p. 1 49).
< 1 00 45 1 0-30
p. 233).
Fosberg (1 953, p. 2).
Jones (1 953, p. 2530).
Table 4. Heights of cha racteristic calcareous eolianites.
Figure 1 8- Sea cliff exposure of eolianite of I sla Mujeres of the Yucatan Pe n i n ::>t J i a . L a rge-sca le c ross beds dip steeply landward (left). Uppermost beds a re almost flat-lying and a re pen etrated by numerous rh izoc retions which weather in rel ief. Caliche crust caps the dune rock. Photo by W. C . Wa rd .
quences of eolian limestones (Fig. 10) or as pocket fillings (Fig. 1 1 ) in numerous regions. In dune deposits of southeastern Australia where paleosols separate two or more se quences of eolian cross-strata, they are described by Bird ( 1 972) as differ ing from wind-deposited sand ( 1 ) in their color - red, brown or yellow of iron oxides, locally black from car bonaceous material, (2) in their tex ture - sandy silt or silty clay, and (3) in the common presence of an
underlying irregular calcrete surface . Australian buried soil zones associated with carbonate dunes are mostly l to 2 ft thick. A comparable paleosol unit in the Thar Desert of India is stated by Goudie and others ( 1 973) to "separate two phases of aridity and represent an interval of significant wet climate conditions of the region. " In both places, numerous fossil root casts are preserved in the dunes below. A considerable stratigraphic section
including five calcareous eolianites and six soil units, each with a local name, is recognized in Bermuda (Sayles, 1 93 1 ). These extensive dunes are attributed by Sayles to times of glaciation when sea level was low and land surfaces correspondingly large, storms were numerous and vegetation sparse; the ancient buried soils were considered to represent interglacial times . They are deep red and com monly a few fe et thick .
Eolian 149
TABLE 5 Locality
Formation or Age
Thickness (in feet)
Reference Ward ( 1 975, p . 51 1 ).
Isla M ujeres, Yucatan West side, Isla Mujeres, Yucatan
Pleistocene ridge
90 + 1 1 8+1
Ward ( 1 975, p. 5 1 1 ).
Isla Cancun, Yucatan
Quaternary
80
Ward and Brady (1 973,
Bermuda
Pleistocene ridge
Bermuda
Quaternary Quaternary
Victoria, Australia West Tripoli, Ubya
Older lithified du"nes Gargaresh calcarenite
Libya Tunisia Abu Dhabi Kathiawar, India Kutch, India
Kathiawar, India Abrolhos Islands. Australia
p. 233).
Sayles ( 1 931 , p. 385). Verrill ( 1 907, p. 49).
" Few hundred"
200-268 Est. 350 +
200 + 90:t 90 9-30 20. 1
Bird (1 972, p. 349). Hoque (1 975, p. 395).
Miliolite Formation
200 1 00
Miliolite Formation
Coast 900; Veraval 1 382
Biswas ( 1 971 , p . 1 49). Biswas (1 971 , p. 1 53). Lele ( 1 973, p. 305).
Dune limestone
30
Late Quaternary Late Quaternary Quaternary, " Unit 1 " Miliolite Formation
Fabricius ( 1 970, p. 757).
Fabricius (1 970, p. 757). Evans and others (1 969,
p. 1 48).
Teichert (1 947, p. 1 52).
1 Height above sealevel 2Borehole
Table 5. Thicknesses of some
characteristic eolian limestone formations.
Relationship to Changes in Sea Level and Climate
Carbonate dune rocks on Bermuda were thought by Sayles ( 193 1 ) to have been deposited during glacial periods, when lowered sea level exposed the inner-shelf sands to wind erosion. Five paleosol zones were recognized by him within the Pleistocene eolianite sequence on Bermuda, and he attributed each of these fossil soils to a period of weathering during times of warm interglacial climates . Dune rocks along the western Australia coast, however, were believ ed by Fairbridge and Teichert ( 1 953) to have formed at different high stands of sea level during warm, arid interstadial phases . These geologists concluded that eolianites may build up during either regressive or trans gessive stages . Eolianites on Ber muda, according to Bretz ( 1960) and
Land and others ( 1 %7), accumulated during interglacial high stands of sea level . A study of dune rocks of Israel prompted Yaalon ( I %7) to caution that climate varies irregularly in space and time, so that neither eolianite nor buried soils by themselves can be used as criteria for specific climatic episodes of the Quaternary. Most Quaternary eolianites are believed by Fairbridge ( 1 97 1 ) to have built up rapidly at the beginning of the regressive phases, when colder temperatures brought increased aridi ty. Thus coastal eolianites are "fine indicators corresponding to the begin ning of a cold period" (Fairbridge, 197 1). Along the northeastern coast of the Yucatan Peninsula, eolianites accumulated during the early stages of the later Pleistocene regression of sea level and again during the max imum transgression of the Holocene
sea level (Ward, 1 970, 1975). In conclusion, the causes or con trols of the alternating sequences of calcareous eolianites and soil zones in Bermuda, India, Yucatan and some other areas have been considered by various geologists (for example, Sayles, 193 1) to be changes in sea level during the Pleistocene. Periodic drops in sea level, attributed to the forming of ice sheets, are considered favorable to exposure of much sand for dune build-up and so, at least in Bermuda, a glacial-interglacial stratigraphic sequence has been recognized.
1 50
Eolian
Figure 1 9-Large-sca le , h ig h angl e foresets dipping landward, c a rbonate eolianite a t Bannerman Point, Ele uthera I sland, Bahamas. Photo by E. A Shinn.
ORIGIN AND CHARACTER OF
CARBONATE SAND Sand Sources
The development and maintenance of a carbonate-sand dune field is clearly controlled by the adequacy of its calcareous sand source and by the presence of a prevailing wind to transport and deposit the sand. By far the greatest source of carbonate sand is along the margins of seas where marine bioclasts, ooids and foraminifers are forming and ac cumulating . Beaches are probably most important as the immediate source of wind-blown calcareous sand; the supply of sand on them is constantly being replenished by longshore currents and, as the sand dries, movement inland is· accomplish ed by onshore winds. Illustrations of carbonate dune sands that are derived from beaches or other shore deposits are numerous. In the Thar Desert of western India, the southwesterly monsoons seasonal ly carry inland much sand from the Rann of Cutch and adjacent seacoast (Wadia, 1 953). Other records from India of calcareous eolian sands, derived from littoral deposits, are by Evans ( 1 900) and by Qadri ( 1 957). Along the Mediterranean coast of Africa , sand similar in textural at tributes and skeletal composition to the present beach sand, suggests that
the extensive eolian limestones of that area were formed from Pleistocene beaches under a dominantly north west wind (Hoque, 1 975). Additional dune regions developed by beach furnished sands are the Bahamas (Ball, 1 967), and Bermuda (Sayles , 1 93 1 ; MacKenzie, 1 964b) . In some regions calcareous dunes along coastal fringes advance far in land. Most notable in this respect is the Thar Desert of India where the wind has transported large quantities of foraminifers inland as much as 400 km or more. The abundance of their skeletons or tests in the sand de creases only gradually (Goudie and Sperling, 1 977). In the Wahiba sands of Saudi Arabia, the carbonate frac tion is thought to have been blown from coastal sabkhas and marine ter races, in some places, for great distances. Interior desert winds also may build up carbonate dunes, although such dunes are relatively uncommon. In the Southern Desert of Iraq, the main source of the eolian clastics is considered to be the Mesopotamian Plain (Skocek and Saadallah, 1 972), where the carbonate content is generally less than 300Jo and "is con centrated in the finest fraction. " Along the Persian Gulf i n Saudi Arabia and Iran, fine sand and dust, some of it highly calcareous, is transported long distances from the
northwest by the prevailing wind (Emery, 1 956) . Other illustrations of interior source areas for carbonate clasts are in the lea Desert of Peru where Newell and Boyd ( 1 955) observed dunes formed largely of Eocene mollusk shell fragments, and in the Great Salt Lake Desert, Utah, where Jones ( 1 953) recorded ooids, gypsum fragments, ostracod clasts, and algal particles all derived from marginal deposits of a former large lake. On the island of Bermuda where Sayles ( 1 93 1 ) made extensive studies of the eolianites, he noted that although they are composed chiefly of pelecypod shell fragments, they contain some foraminifers, fragments of crab shells and algae. They have an insoluble residue that includes eight principal minerals as determined by E. S. Larsen (Sayles , 1 93 1 ) . These minerals are of special interest because, although pyroxene, perov skite, magnetite, and glass were prob ably derived from the volcanic plat form underlying the island, the quartz, orthoclase, zircon , rutile, and tourmaline are typically continental minerals lacking satisfactory explana tion for their presence. Character of Sand
The character of sand in carbonate dunes in most places closely resembles that of adjacent beaches or shallow
Eolian
151
Figure 20- Low-angle wedg e-planar cross beds, Pleistocene eolianite, I s l a Contoy, Yucatan Pe n i nsula Photo by W .
C. Wa r d .
marine areas that serve as principal sources of clastic sediment. This similarity of grains between wind deposited sands and near-shore or beach sands is illustrated in Libya and elsewhere along the coast of north Africa as described by Glennie ( 1 970) . Similar comparisons are made by Ball ( 1 967) for eolianites of the Bahamas , by Harms and others ( 1 974) for dunes of coastal Yucatan, and by MacKenzie ( 1 964b) for Ber muda dune limestones. Among marine clastic sediments, the principal ingredients that con tribute in various proportions to the forming of eolian deposits are: ( 1 ) the fragments of shells, usually referred to as skeletal material ; (2) the tests of foraminifers which may be transported for considerable distances by the wind; (3) oolitic accumulations
which , in favorable environments, form vast masses of pure carbonate sediment; and (4) peloidal grains developed from the erosion of earlier limestones . All four of these grain types may occur together in a calcareous eolianite, but the propor tions of each varies widely from one locality to another . Skeletal material - Fragments of skeletal material (bioclasts) form most of the eolianites in some regions (Fig. 1 2B) . In coastal deposits of Libya, calcium carbonate which constitutes from 75 to 8 8 0Jo of the sediment is described as "coquinoid material" by Hoque ( 1975). In Bermuda, the dune sand in some places (Verrill, 1 907) is said to consist "almost entirely of broken shells," and in Baja Califor nia, eolian deposits are reported to be composed of "rounded to subround-
ed shell fragments" (Anderson, 1 950). In some regions, mixtures of numerous other types of skeletal material occur with shell fragments as, for instance, on the west coast of Australia where fragments of echinoidea, corals, foraminifers and bryozoans are listed (Teichert , 1 947; Fairbridge, 1 950) as being associated with molluscan shells. In western In dia, the eolian sands are described (Chapman, 1 900) as a mixture of oolites, foraminifers, excretory pellets and echinoderm spines, with clasts from mollusks. Also common in some deposits are fragments of algal crusts or stromatolites. Foraminifera - Certain small marine animals, the shells of which commonly are worn but not broken, may form major parts of some car bonate rocks. Foraminifers are a
152
Eolian
··LEGEND�
.._ ...._ � IMP
�
-·
..
_._.... _
-
..lutlon
.-....
�:= t(:r,�
A
�150' -1
Figure 2 1 -General ized structures in a Pleistocene carbonate deposit alon g East Street road cut on New Providence Island, Bahama Islands. The foreshore ca rbonates on the left are younger and have transg ressed over part of the calcareous dune. The entire sequence is now lithified. Tabu lar-planar c ross-beds in lower part were formed by paleowinds from the north, c ross-strata above were by paleowinds from the east' or west. Field sketch by T. S. Ahlbrandt.
good example. In western India, the abundance of their tests in the Miliolite Limestone, which is con sidered eolian by many geologists, has been referred to by Shrivastava ( 1 968) and Lele (1973); the name applied to that limestone refers to the abun dance of miliolids included in it . In Baja California, 15 species of foraminifers of open-ocean types were recorded (Phleger and Ewing,
�
---.
A'
1 962) in a single sandy calcareous eolianite. In Bermuda , 3 4 species of foraminifera were found in one specimen of fine-grained eolian sand reck (Sayles , 1 93 1 ) . I n regard to the generally good preservation of foraminifers , Glennie ( 1 970) stated that they are commonly better preserved among coastal wind deposited sands than in deposits of marine environments. Furthermore, foraminifers are susceptible to long distance transport by the wind, as shown by Goudie and Sperling ( 1 977) , who described the common occurrence of their tests in at least two locations in the Thar Desert of India about "400 km from the nearest sea" source. Also significant is the record of largely calcareous dunes, containing much foraminiferal material that was accumulated in a
temperate climate as represented on the Galway Coast of Ireland (Evans, 1 900) . Carbonate dunes composed dominantly of foraminifer tests are especially characteristic of wind blown deposits on islets of the Pacific Atolls where beaches largely consist of several species of these animals. Examples of such dunes are cited by Cloud ( 1 952). High proportions of foraminiferal sands are also recorded in various parts of limestones at tributed to eolian origin near the coast of western India - Kathiawar, Kutch , and elsewhere - as described by Shrivastava ( 1 968), Biswas ( 1 97 1 ) , and others . Numerous tests of foraminifers, mainly rotalids and miliolids, "mixed with quartzose sand' ' are reported far inland in the Thar Desert of India by Goudie and
Eolian
1 53
Fig u re 22-La rge-scaie trough cross strata in carbonate eolianite, Castle Rock, Bermuda. Photo by E. A Shinn.
Sperling (1 977). Some fusulinid tests occur in most other regions of car bonate eolianites including Australia, Bermuda, Baja California, and the Mediterranean coasts. The temperate climate calcareous eolianites of Galway, Ireland , are recorded as "largely calcareous and contain much foraminiferal material" (Evans, 1 900) . Other marine animals Among marine animals, other than foraminifers, characteristically found in coastal eolian carbonates are gastropods and ostracods . Both of these groups are reported by Shrivastava from the Miliolite Limestone near the coast of Saurashtra in India (Shrivastava, 1968) and are said to be similar to animals now living in the nearby sea. The gastropod Strombus bubonius is recorded (Hoque, 1 975) from eolian limestone on the Libyan coast and from Tunisia (Fabricius, 1 970). This species is said to be " now endemic in tropical waters off the Senegal Coast and the Canary Islands, but not in the present Mediterranean. " Terrestrial animals - Carbonate dunes and eolian limestones common ly contain the skeletal remains o f land animals. Terrestrial gastropods are especially numerous as noted on Ber muda (Sayles, 1 93 1 ) , the Bahamas (Ball, 1 967}, Great Salt Lake Desert -
(Jones, 1 953), and elsewhere. In "false-bedded blown sands" near Cape Town , South Africa, eolian deposits contain, in places, the bones of land animals among masses of comminuted marine shells (Rogers and Schwarz, in Evans, 1 900) . In Vic toria, southeastern Australia, an ex tinct species of giant kangaroo is recorded by Bird ( 1 972) as con tributing evidence of the age of the eolian limestone containing it . Ooids - Ooids are a most impor tant source of carbonate material in many regions of dune formation. They commonly imply a nearby en vironment of deposition consisting of shallow, agitated waters, though they may have accumulated from the weathering of oolitic deposits of an earlier age. Areas in which ooids have been recorded as a principal compo nent of eolianites are the coast of Yucatan (Fig. 1 3) , where they form a belt of clean carbonate sand (Ward and Brady, 1 973 ; Ward , 1975), the Mediterranean coast of Tunisia (Fabricius, 1 970), where the "ooids are reworked from submarine and on shore oolites , " and the coastal plain of Egypt , where ooids have probably been formed in agitated marine waters (Selim, 1 974). Table 1 of Selim ( 1 974) summarizes the constituent composition of the limestones of several ages represented in the Egyp-
tian area and shows that ooids form more than SOo/o of all constituents in the first two limestone ridges. Ooids are also important constituents in the eolianites of western India as discuss ed by Chapman ( 1 900), by Shrivastava ( 1 968) , and by Biswas ( 1 97 1 ) . A most unusual occurrence of oolitic eolianite, however, is in the Great Salt Lake Desert where ooids, derived from lake waters of the former Lake Bonneville, are associated with gypsum crystals, fine quartz grains and some shell frag ments in a cool-climate but desert en vironment (Jones, 1953). Carbonate dust particles
-
Although in most desert regions dust particles are largely removed from dune deposits and carried away in the atmosphere, carbonate dunes and eolian limestones in some areas are reported to contain appreciable amounts of CaC0 3 in the cryp tocrystalline form . A sample of dust carried over the Persian Gulf by winds from the northwest was record ed by Emery (in Graf, 1 960) to con tain "83 percent calcite. " In the Southern Desert of Iraq , mainly along the western margin of the Euphrates River alluvial pl ain , dunes are formed where "travelling sandy materials are brought to a halt . . . due to higher air humidity" (Skocek and Saadallah, 1972).
154
Eolian
Figure 23-Trou g h c ross beds on flank of Ho:ocene dune, Isla Cancun, Yucatan Pen i nsu la . Photo by W. C. Ward .
Although the carbonate content in these deposits is mostly within the 1 0 t o 1 5 0Jo range, the amount o f car bonate "decreases with increasing grain size," suggesting that a major source is from desert soils and alluvial sediments of the Mesopota mian Plain . Mixed grains A mixture of quartz and other nonsoluble grains with carbonate grains occurs in various proportions in many eolianites (Fig. 6). In western India, Biswas ( 1 97 1 ) recorded that coastal deposits are free from quartz, but that inland deposits contain 60 to 900Jo quartz. Quartz, plagioclase, ig neous rock material and ferruginous casts, associated with carbonate material in India, are also described (Evans, 1 900). Along the Arabian -
coast, dune sands were determined by Evans and others ( 1 969) to include 25 to 40% polished, well-rounded quartz and feldspar grains in sediment com posed dominantly of calcium car bonate clasts. In southeastern Australia, carbonate dunes were calculated (Bird, 1 972) to consist of 50 to 90% calcareous sand with the remainder mostly quartz.
reflect the attributes of beaches or nearshore marine environments from which their sands were derived . Thus, most eolianite deposits are composed of fine to medium, well-sorted and moderately to well-rounded carbonate grains, but exceptions resulting from abnormal source materials, different distances of transport, and mixing of depositional agents, occur in numerous areas .
TEXTURAL ATTRIBUTES
Grain Size Principal Properties of Grains
The principal textural properties of carbonate sand in eolianites are grain size, sorting, degree of rounding, grain surface features, and orienta tion of grains. Mostly these properties are similar to those represented in nearby source areas because they
Fine and medium grain sizes are most commonly represented in eolian carbonates (Table 6) . Considerably coarser grains, formed as lag concen trates under strong wind conditions, are described from the lea Desert , Peru (Newell and Boyd , 1955). These lag deposits were derived largely from
Eolian
1 55
TABLE 6 Size Locality
Very Coarse Coarse Medium Fine
Bennuda
Very Fine
X X
Bahamas Yucatan
X
X
Yucatan
X
X X
Baja California
Onotoa Atoll
X
Ubya
X
X
X
Abu Dhabi
Kutch, India
X
Thar, India
X
Victoria. Australia
X
X
X
X
Reference MacKenzie (1 964, p. 53b). Ball, (1 967 p. 573). Ward and Brady (1973, p. 233). Hanns and others (1974, p. 144). Phleger and Ewing (1 962, p. 1 62). Cloud (1952. p. 58). Hoque (1 975, p. 395). Evans and others (1969. p. 1 48). Biswas (1971, p. 1 54) . Goudie and Sperling (1977, p. 630). Bird (1972, p. 350).
Table 6. Dominant or mean grain size. calcareous eolianites.
disintegrated mollusk shells of Eocene
the sea from an inland source of car
to "the i nfiltration of fine fract ions
age . Other very coarse eolian car bonate sands are in the Great Salt Lake Desert, Utah, where 1 12- to
bonate sediment such as a caliche sur
transported by dust storm s . "
l -inch layers of granule-size algal fragments alternate "with thin layers of gypsum-arenite" sand (Jones, 1953). These beds of coarse grains were postulated to be indicators of "repeated intervals of maximum wind velocity . " At t h e other extreme, some wind
face or a calcareous desert fan , it may carry much calcareous dust . Prevailing northwest winds over the
Grain Sorting Virtually all reports on eolian car
Persian Gulf, for instance, were recorded by Emery (in Graf, 1 960) to
bonate sands agree on the excellence of their sorting. Calcareous dune sands i n the Bahamas are described as "characteristically well sorted , " (Ball,
carry dust containing 830Jo calcite. Analysis of a Persian Gulf dust sam ple (Emery, of
1 .6
1 956) showed 78.9 silt,
fine sand ,
percentages and
1 9.7
deposited sediments contain ap
clay. Deposits o f such carbonate dusts are recorded from lran (Skocek
preciable amounts o f carbonate
and Saadallah , I 972) where a relative
materials in the form o f dust par
ly widespread bimodal grain-size
ticles. Where wind is blowing toward
distribution in the dunes i s attri buted
1 967).
Along coastal Yucatan, deter
minations of
29
samples showed
28
of
them to be very well to well sorted (Ward and Brady, 1 973) and i n car bonate dunes of Kutch , India, " a very high degree of sorting" i s believ ed to be i ndicated by the uniformity
1 56
Eolian
Figure 24-Trough c ross-bedding form ed in spil lover deposits of ca rbonate eolianite, Banne rman Points, Eleuthera I s land, Baham<Js. Photo by E. A . Shinn.
F i g u re 25-Deformational structures i n avalanche sand , beve l led by p l a n e o f sl i p-fac e ; eol ian ca rbonate roc k ; Banner man Po i n t , Ele uthera I s l a n d , Bahamas. Photo by E. A. Shinn.
in shape and grain size (Biswas, 1 97 1 ) . Likewise, in Australia (Bird, 1 972), along the Persian Gulf (Evans et a! , 1 969), and in numerous other localities , calcareous dune sands are recorded to be well sorted. Roundness of Grains
Carbonate sand grains are eroded by wind action two to four times as fast as are quartz grains, according to Glennie ( 1 970) . Therefore, one might expect most carbonate eolianites , if relatively pure in CaC0 3 composition and if derived from a source such as a beach where abrasion has been ac-
tive, to have well-rounded grains . A corollary to this generalization is that dunes of mixed carbonate and quartz sands may contain mixtures in grain roundness. The validity of the foregoing generalization is supported by the studies of Evans and others ( 1 969) in Abu Dhabi on the Persian Gulf where they found well-rounded grains of comminuted shells, but only moderately rounded grains of quartz and feldspar , and by the work of Shrivastava ( 1 968) on the miliolite rocks of the Saurashtra coast in India which consist of quartz grains generally subangular to subrounded, whereas the associated shell fragments and ooids are rounded and polished. Roundness of some grains may have been inherited and actually represents the work of beach processes . Surface Features
Because of their CaC0 3 composi tion, grains in calcareous dunes or eolian limestones do not commonly develop conspicuous surface features such as high polish, frosting, or pro minent scratches. Well-rounded, polished grains, formed from com minuted shells, are reported from Abu Dhabi by Evans and others ( 1 969) , but , in general, such features have not been mentioned for most areas . Tests of foraminifers and tiny gastropod and pelecypod shells in
western India are described (Biswas , 1 97 1 ) as "worn-out" from having been rolled and abraded during long wind transport . A detailed study of polish on car bonate grains of the Yucatan coast has been made by Folk ( 1 967) who concluded that, like rounding, polish was largely the direct result of wave energy and, for that area, furnishes no evidence of chemical polish . Although dune deposits were not specifically mentioned, the beach sources of eolian sands on windward sides of the islands displayed highest polish and, on grains along the lee sides, polish was lacking. Two other factors seem to have controlled the amount of polish on the Yucatan sands: ( I ) Grain size - There was essen tially no polish on grains "coarser than about 50 mm or finer than about 0. 1 5 mm"; and (2) Type of grain - " Dense black limestone polishes more easily than coral fragments and these polish more easily than Halimeda flakes . Skewness
Little attention seems to have been given to the attribute of skewness in the sand of carbonate dunes, and probably little signi ficant information can be deduced from this property . Skewness determinations were made for sands of the Libyan coast (Ho-
Eolian
1 57
Figure 26-Photomicrograph of
meniscus cement in Holocene eol ianite, I sla Can c u n , Yucatan Pe ninsula. Plane light. Bar scale is 200tL
que, 1 975); 1 6 samples showed 9 to be negative and 7 positive. Skewness values ranged from + 0.21 to - 0. 3 4 with a mean skewness that was nearly symmetrical . SEDIMENTARY STRUCTURES Cross-Stratification
The composition and texture in clastic rocks give evidence concerning the source and the mode of transpor tation, respectively, but the sedimen tary structures, because they are formed during accumulation of grains, best record the environment of deposition . In carbonate dunes and in the eolian limestones that develop from them, cross-stratification is the most characteristic and conspicuous structure. The principal types of cross-strata and strata that are recognized in quartz sand dunes are tabular- and wedge-planar, festoon trough-type, and horizontal types. These also occur in carbonate dunes but in different proportions . Types of cross strata A majority of carbonate dunes are formed along coasts where onshore winds develop ridges parallel to the strandline (Fig. 15). Thus, a planar structure with slipfaces dipping landward is a com mon form (Figs. 16, 1 7 , 1 8, 1 9) . On -
the coast of Israel such structures are described (Yaalon , 1967) as forming straight parallel sets, several meters long . On the Mediterranean coast of Libya, mostly tabular-planar cross beds occur (Hoque, 1975), and in the Bahamas , at numerous places a "single huge sand wave [may form] one very large set of foreset cross beds , " that probably consists of tabular-planar structures . They are well illustrated for Bermuda in several figures by Verrill ( 1 907, Figs. 19, 22, 29) and by MacKenzie ( 1 964a) . Wedge-planar structures are likewise well represented in the com mon transverse-type dunes on many coasts (Figs. 14, 20). In coastal deposits o f western India "mostly wedge-type current bedding" is char acteristic, according to Biswas ( 1 97 1 ) and from the Yucatan coast "dominantly wedge-shaped rather than tabular" cross-strata are describ ed by Harms and others ( 1 974). Wedge-planar structures range in height from a few to 10 m or more, as seen in sea cliffs of Isla Mujeres. Even the inland dunes in the Great Salt Lake Desert apparently are remarkably uniform in orientation, and consist of wedge planar cross strata as illustrated in Figure 3 of Jones ( 1 953).
Figure 27 -Photomicrograph showing grai n-contact (meniscus) cement in Holocene eol ianit e , Isla Can c u n , Yucatan Pe n i nsula. Crossed nicols. Bar scale is 200�t.
A distinctive type of structure that has been recorded from numerous carbonate eolianites is the trough-type that forms a festoon pattern (Figs. 21 , 22, 23). Such structures are un common in the dunes of inland
158
Eolian
TABLE 7 Reference
Dlp Foreset
Salt Lake Desert.
Jones (1953, p. 2532).
maximum
Bermuda
MacKenzie
Locality
..
Utah
(1964a,
Kutch,
Kathtawar,
India
Libya
p. 144 9). Biswas (1971, p. 155).
5, Hoqp �197 . .
Western
F.airbridge (1950, p. 41).
Northeastem
Ward
Australia
Table 7. Angles of dip recorded for foreset and tor windward-side laminae in various dune fields.
deserts , yet they are reported from carbonate coastal dunes of Libya (Hoque, 1975), Yucatan (Ward, 1 975), Bermuda (MacKenzie, 1 964a) and the Bahamas (Ball, 1 967). Unfor tunately, little "has been recorded about the size and character of these structures. They are "of various scales" and "occur within dune cores" according to Ward; they develop in saddles between "spillover lobes" (Fig. 24) in dune ridges as described by Ball and by MacKenzie. How they compare with the festoon troughs of meandering streams (Frazier and Osanik , 1 964; Bernard and Major, 1973) or in braided rivers (Smith, 1970) is not known . Horizontal strata are recorded from some carbonate dune complexes. In north African coastal deposits, such strata are said by Hoque ( 1 975) to be present , but not common; he did not indicate whether they were "wind ward topset beds" or interdune deposits. Angles offoreset dips - Most foreset dip angles that have been recorded for carbonate eolianites show features similar to those of dunes formed from quartz sand. In general, foresets of the slipfaces are
)
Yucatan
�1975,
p . 5 8).
high angle (Figs. 1 5 , 16, 1 8 , 19), ap proaching the angle of repose for dry sand or about 3 4 o , and those of the windward side are low angle or less than 1 5 ° . Measurements that have been recorded from some areas of detailed study are shown in Table 7. The foreset dips for Libya given by Hoque (1975 , Table 1) seem low for dune slipfaces, but these are only mean values; maximum values are not given . These values are based on 321 readings and include 559Jo that are high angle or greater than 20° , and 450!o that are low angle, less than 20° . Measurement of strata - Quan titative data on the dimensions of strata, and of sets or cosets of car bonate eolianites are scarce . The length of foresets in Bermuda eolian limestones is given as from l to 50 ft by MacKenzie ( 1 964b); for coastal Yucatan they are merely referred to as "large scale" that "dip steeply" (Ward, 1975). The thickness of sets has also received little attention, although this is one feature that can easily be com pared with structures in ancient rocks. For Yucatan the thickness of sets is stated (Harms et al, 1 974) to
Dip Up-wind
30.32
(degrees) -
30·35
1 0-1 5
(degrees)
Very gentle
(>5)
-
to 20-30
Average for 24
-
localities 1 7-27 32
28·39
maximum
<10
range from a few to 10 m or more. The mean thickness of units (sets) on the Libyan coast is given as 148 em and they range from 77 to 308 em (Hoque, 1975). Sets and cosets on Bermuda are recorded as ranging from 1 to about 75 ft (MacKenzie, 1 964b). Traces offoresets - Considerable attention has been given in some cross-strata investigations to the pro file shape and the orientation of foresets. A high-angle, sloping sur face may meet the underlying plane at an acute angle, but more frequently it forms a tangent with the base (MacKenzie, 1 964a ; Ward, 1 975) . The cause of the tangential development is probably the same as in quartz sand deposits - either the sand is poorly sorted and the finer grains form bot tomsets, or an appreciable amount of sand is deposited as fallout from suspension at the front of the ava lanche sediment . A second distinctive feature of the foreset trace is its curvature as seen on a vertical plane parallel to the wind direction. A large majority of all foresets, both eolian and aqueous, are concave upward or are nearly straight, because of the nature of set-
Eolian
1 59
Fig u re 28-Photomic rograph showing pendulous cement i n Holocene eol ianite, Isla Blanca. Yucatan Peninsula. Plane
light. Ba r scale is 209/l·
ding at or near the angle of repose. In carbonate dunes, the occurrence of some foresets that are convex upward has been noted (Hoque, 1 975; MacKenzie, 1 964a). This upward con vexity has been attributed by MacKenzie ( 1964a) to early stabiliza tion of the dunes and, therefore, is considered a feature distinctive of calcareous sand . Similar structures, however, are found at White Sands, New Mexico, in gypsum dunes (McKee, 1 966) and in the Killpecker field, Wyoming, in siliceous dunes (Ahlbrandt, 1 975). In each place they occur only in parabolic dunes where the bases of projecting fronts or noses are oversteepened. Convex upward foresets, therefore, are be lieved to be a function of undercut ting by crosswinds on the noses of parabolic dunes and are not control led by sand composition . In Libya, traces of foreset beds, on horizontal planes cutting carbonate dunes, were noted by Hoque ( 1 975) to be "strongly arcuate" and were in terpreted by him to represent festoon or trough structures. Azimuth patterns and paleocurrent directions - Mean dip-direction vec tors and the amount of spread have been determined for cross-strata dips in a few areas of eolian carbonate
distribution. Cross-stratified limestones near the coast of Libya were determined (Hoque, 1 975) to have a mean azimuth of 1 0r and a standard deviation of 80° . The vec tors had a bimodal distribution with modes in the northeast and southeast sectors. Similar studies on the Yucatan coast (Ward, 1 975), based on 205 foreset measurements, showed a spread in dip directions mostly within arcs of about 90° . On the Bahamas , Ball ( 1 967) determined that cross-bed dips in "spill-over" lobes, scattered along dune ridges, generally range through "an arc of about 1 80° centered onto the platform . " All of these studies seem to confirm a fair consistency of sand transport direc tion in the dune fields.
Thickness of laminae and beds - The laminae in calcareous
dunes seem to be similar in thickness, extent, and sorting to comparable siliceous dunes of similar en vironments . In general , sorting is ex cellent where appreciable sand has been transported . Experiments with colored sands (McKee, Douglass and Rittenhouse, 1 97 1 ) show that on a dune slipface, sand will sort into laminae of distinct size grades in the distance of a few feet . On the other hand, because a majority of
Figure 29-Photomicrograph showing eolianite grains cemented on the bot toms (pe ndulous cement) and partly di ssolved on the tops. Ple istocene eol ia nite , Isla Mujeres off the Yucatan Peninsula. Crossed n icols. Bar scale i n upper r i g h t is 200�-t.
calcareous dunes are formed close to the source of material which com monly is a beach or other nearshore deposit , extraneous large particles such as unbroken shells, coral fragments, or beach trash may be in-
1 60
Eolian
Figure 3D-Photomicrograph showing needle-fiber c eme n t i n Ple istocene eol ian ite . Isla Mujeres off the Yucatan Pen i nsu la . C rossed nicols . Bar scale is 1 00;.t.
large scale of the cross-beds are features that are unusual in marine carbonates of the area. Minor Structures
Ripple marks Ripple marks con stitute a diagnostic, but relatively un common structure in eolian limestones. Although these structures frequently cover the windward side and, occasionally, the leeward side of modern calcareous dunes in the same manner as on active quartz sand dunes, the ripple marks are seldom preserved. Migration of dunes tends to destroy nearly all strata deposited upwind; only those relatively uncom mon ripples to lee which are buried by avalanching sand have a good chance of preservation. This selective preservation of lee-slope sediments has been described by McKee ( 1945) with reference to the Permian Coconino Sandstone of Grand Can yon and seems to apply to most dune deposits. Two characteristics of wind-formed ripple marks seem to be diagnostic features and serve to differentiate them from subaqueous ripple marks. One of these features, first noted by Kindle ( 1 917) , is the ripple index, or ratio of height to wave length. Wind ripple marks are characteristically low and therefore have high indices. The degree of sorting in the sand controls -
Figure 3 1 -SEM photomicrograph of needle-fiber cement in Ple i stocene eolian ite from I s la M u j e res off the
Yucatan Pen insula.
corporated . An alternation of finer- and coarser-grained carbonate laminae is typical of eolian sands, according to Glennie ( 1 970) and this seems to be borne out by the long, even cross strata that are etched out in most dune peels . Dune strata in general are relatively thin. In Bermuda, they are described (MacKenzie, 1 974a) as ranging from 2 mm to several em . In the Great Salt Lake Desert, oolitic dune foresets are reported (Jones, 1 953) to be between 2 and 8 mm, although granule-size fragments of algae locally form layers ranging from 1 .3 to 2.6 em in thickness, that alternate with the thin foresets. Dune cross-strata in the Bahamas are recorded by Ball ( 1 967) as being "paper thin . " He pointed out that both this thiness of laminae and the
the index, according to Bagnold ( 1 943) who conducted experiments in this field and concluded that the ratio for very uniform sand grains "was usually l to 70 and never more than 1 to 30." The second diagnostic feature of eolian ripple marks is their characteristic orientation - parallel to the dip directions of the high-angle foresets on the leeward side of dunes. This feature, which is seen as a series of parallel ridges and troughs on the surfaces of cross-strata, is distinctive and wherever preserved on dune rocks is a good indicator of genesis (McKee, 1 945). Relatively few ripple marks on car bonate eolianites seem to be recorded in geologic literature. Along the Yucatan coast, dune deposits are described as "abundantly cross stratified and rarely ripple-marked" (Ward , 1975). In Bermuda, ripple marks have been noted, along with root casts and other minor structures (MacKenzie, 1 964a). Coarse sand rip ple marks (0. 1 5 to 0. 30 m high; 1 .0 to 1 . 5 m apart) in Peru are described by Newell and Boyd ( 1955) . These rip pled sands are considered a product of weathering and deflation and finer fractions have been selectively win nowed; locally, they are formed of fragments of fossil mollusks of Eocene age (Newell and Boyd, 1955).
Eolian
1 61
Figure 32-Photomic rograph showing mic rocrysta l l i n e-rind and needle-fiber cements in Pleistocene eol i anite, Isl a Mujeres off the Yucatan Peninsula. Peneropl id forami nifera (center) and Halimeda (upper right) are penetrated by small borings (rootlet borings?). Crossed n i cols. Bar scale is 1 00�<.
On the coast of Israel, well-developed ripples with coarsest particles on crests are reported by Yaalon ( 1 967). Breccias and break-aparts
Sedimentary structures in dunes, com monly referred to as break-apart laminae and breccias, have been reproduced in the laboratory and studied experimentally (McKee, Douglass , and Rittenhouse, 1 97 1 ) . As shown in Table 1 , column g , of that report , the development of such structures seems definitely to be a function of the degree of cohesiveness in the sand body. Wet sand is respon sible for most breccias; wet crusts on ' dry sand commonly shatter i nto break-aparts. Both phenomena were found to be common in some quartz sand dunes on the wet Brazilian coast (McKee and Bigarella, 1 972). In carbonate dunes and eolian limestones, breccias and break-aparts have been noted in some places . This association is to be expected as many eolian carbonate deposits are ac cumulating in relatively humid coastal areas. Brecciation occurs within and between eolianite units in Bermuda (MacKenzie, 1 964a) , and breccia, composed of "altered fragments of eolianite," is said to occur at several horizons in sections on the Bahamas
(Ball, 1 967). A caliche breccia is also reported in Yucatan and is believed to be related to the development of caliche surfaces (Harms et a! , 1 974). On the Bahamas it occurs locally at cliff bases (Fig. 1 8). Contorted bedding Contorted bedding of various types is a common feature of many dunes, both car bonate and quartz-sand types, especially in areas of high humidity and appreciable rainfall that are responsible for slumping on high angle slipfaces (Fig . 25). Numerous varieties of contorted bedding have been recognized and described through laboratory experiments (McKee, Douglass, and Rittenhouse, 1 97 1); most of these types were observed in wet coastal dunes o f Brazil (Bigarella, Becker, and Duarte, 1 969; McKee and Bigarella, 1 972) . Contorted bedding in carbonate dune deposits has been recorded from Libya where it was described as "penecontemporaneous slump struc ture" (Hoque, 1 975) and from Yucatan where it was attributed to "overload slumping on the dune slopes" (Ward, 1 975). On the North African coast , the " foresets [of cer tain eolian limestones] exhibit crum pling of some laminae . . . very sug-
gestive of slumping seen on the flanks of recent dunes from nearby" (Glen nie, 1 970). Rhizocretions or root traces The term "rhizocretion" was proposed by Kindle ( 1 923) and "dikaka" by Glen nie and Evamy ( 1 968) for root cast structures of dune vegetation , preserved in the form of caliche (Figs. 9, 1 2) . The term "calcrete rhizocon cretion" has been used more recently, apparently for the same structures, by Sherman and I kawa ( 1 958) and by Ward ( 1 975). These root systems are apparently common in calcareous eolianites, as well as in other dune deposits (Figs. 10, 1 6). They have been recorded from Bermuda (MacKenzie, 1 964a), the coast of Yucatan (Harms et al, 1 974; Ward, 1 975), the coast of Libya (Glennie, 1 970), and many other dune areas . They are described by Harms and others ( 1 974) as zones "of thicket-like root casts," and by Glennie ( 1970) as "cemented plant root molds . " Root systems of eolianites in southeastern Australia are described by Bird ( 1 972) as cylindrical calcrete forms and they commonly mark the presence of paleosols among eolian calcarenites. In Western Australia, root structures that are concentrically-
1 62
Eolian
F i g u re 33-Rh izocretions wea t h e r i ng in
relief from weakly cemented Upper
Pleistocene eolian ite on Isla M ujeres off the Yucatan Pe n i nsu la . Photo by W. C. Ward .
filled calcium carbonate casts are said to be extensive (Teichert, 1 947). In Rajasthan, India, numerous root casts within the calcreted dunes are reported to be 56 to 680Jo calcium car bonate (Goudie et al, 1 97 3) . The presence of a solid core of caliche ' 'in most of the branching and anastomosing rhizocretions" in Pleistocene eolianites of Yucatan has been described in detail by Ward ( 1 975). Root casts with hard, hollow sheaths are also present , but less com mon. In Holocene deposits, hard-core rhizocretions are rare, although a few horizontal and vertical root casts have formed where cementation has filled root holes. Small tubules of microcrystalline calcite, also in Pleistocene eolianites of the Yucatan coast , have been studied with care and described by Ward ( 1975). These tubules are re ferred to as "root-hair sheaths" and have been variously attributed to three possible origins: algal filaments , extensions from the cells o f rootlets, and fungal hyphae. Ward ( 1 975) in terpreted them as most likely the precipitates around root hairs of dune plants. Bioturbation by animals Biotur bation, defined as the disruption and mixing of sediments by organisms, -
has been shown to be a common feature in many dune fields, especial ly those of inland deserts (Ahlbrandt, Andrews, and Gwynne, 1 978). In car bonate dune fields, most of which are in coastal areas, bioturbation by plant root systems seems to be common in many regions. The root structures, as stated before, are referred to as rhizocretions (Kindle, 1 923 ) or dikaka (Glennie and Evamy, 1968). Faunal bioturbation, in contrast, seems to be fairly uncommon or has not been recognized in many carbonate dunes. Referring to the Bahama dune sands, Ball ( 1 967) stated that "bur rowing is rare in wind-blown car bonates ," whereas root casts are quite common . In western Australia, "insect puparia" and fossil "co coons" in solution pipes, together with abundant root casts, are record ed by Fairbridge ( 1 950). Borings in carbonate sands are mentioned by Glennie ( 1 970). In general, however, references to burrows in calcareous eolianites are rare, possibly because an actual scarcity occurs as a result of early diagenesis and development of a cemented crust making an un favorable habitat . Possibly, also, the explanation is that " many geologists are reluctant to associate burrows with eolian deposits" as stated by
Ahlbrandt , Andrews, and Gwynne ( 1 978) in connection with interior dunes. Distinguishing between root molds (rhizocretions) and the burrows or borings of certain animals that ce ment or agglutinize the sand tubes with secretions is not always easy . Various features of shape and distinc tions in color resulting from selective oxidation have proven useful, however, for differentiating between root molds and animal burrows as described by Ahlbrandt , Andrews , and Gwynne ( 1 978), and by Ball ( 1 967). EARLY -DIAGENETIC F ABRICS1
Eolianites are characterized by early-stage cements and other diagenetic fabrics that form in the vadose zone. Meteoric water per colating through carbonate dunes becomes supersaturated with respect to CaC0 3 through the partial dissolu tion of the aragonitic and Mg-calcitic grains . Much of the CaC03 taken in to solution is reprecipitated as low Mg-calcite cement in the pores of the dune sand. Therefore, carbonate ' "Early-Diagenetic Fabrics" was prepared by W. C. Ward.
Eolian
1 63
Figure 34-Photomicrograph of
Pleistocene eolianite with
m i c rocrystall ine-rind cement and m ic rocryst a l l i n e-calcite root-h a i r sheaths i n intergranular pores. I sla Mujeres off the Yucatan Peninsula. Crossed nicols. Bar scale is 2001-'.
dunes may be lithified almost as soon as they accumulate. The early diagenetic fabrics of eolianites are related , in large part , to rainfall , evaporation, and transpiration by dune plants, pene-contemporaneous with deposition . Early-Stage Cements
The "vadose-character" of most early-stage cements in eolianites is easily recognized . One diagnostic characteristic is variability (Dunham, 1 969). Cements in Quaternary eolianites tend to be highly variable in size, shape and distribution, even within an area of a few millimeters in a single rock. Fine-grained layers tend to be more completely cemented than coarse-grained layers. Furthermore, cement types among eolianites of dif ferent ages or locations may be dif ferent , particularly in crystal size (Ward, 1 973 , 1978). In addition to this typical variability, early cements in eolianities are characterized by the predominance of vadose types, such as meniscus, pendulous, and needle fiber cements , and the absence of fibrous grain-rimming cements. Grain-contact and meniscus cements - In general , cement in an
eolianite is preferentially precipitated at grain contacts, where surface ten sion holds droplets of percolating meteoric water. The grain-contact ce-
ment may be microcrystalline, but is generally finely to coarsely crystalline calcite. The growth of grain-contact cement is restricted by the meniscus or outer limit of water held in the corners of pores (Dunham, 1 97 1 ) . As crystal growth proceeds , the pore face of the cement takes on the shape of the meniscus, resulting in rounding of the corners of the pores. Meniscus ce ment (Figs. 26, 27) is well developed in many eolianites . Pendulous cement Calcite ce ment preferentially precipitated on the bottoms of grains is common in some eolianites (Figs. 28, 29). This cement type, commonly called "pen dulous cement, " is widely accepted as evidence of vadose-zone cementation. In Quaternary eolianites of Yucatan the percentage of grains with pen dulous cement is highly variable, ranging from zero in some layers to as much as 65 0Jo in other places. Needle-fiber cement Needle-like crystals of calcite cement (Fig. 30) are common in Pleistocene eolianites of Yucatan , particularly in proximity to ancient weathered surfaces and rhizocretions . This " needle-fiber ce ment" (Ward, 1 970), "whisker crystals , " (Supko, 1 97 1 ) , and "needle-fibers , " (James, 1 972), may be as indicative of vadose-zone cementation as are meniscus and pen dulous cements. -
-
This cement occurs both in in tergranular pores and in intraparticle dissolution voids as unoriented , straight-fibers as much as 200 m long and usually less than 4 m wide. The needle-fibers are composite crystals composed of strings of imbricated flattened rhombs of calcite (Fig. 3 1) . Microcrystalline-rind cement - I n some eolianites, grains are coated with irregular rinds of microcrystal line calcite ( 1 to 2 �-tm) that grades poreward into bladed and rhombic crystals of calcite less than 25 �-tm in greatest dimension (Fig. 32) . This ce ment type in eolianites of Bermuda is called "grain-skin cement" (Land et al, 1 967) . In rocks cemented with finely crystalline calcite, few inter granular pores are completely filled. Pore-occluding cements In the Holocene eolianites of Yucatan, scat tered pores may be totally filled with calcite cement, but adjacent pores re main cement free (Ward , 1 975). Pore occluding cement has a wide range of crystal sizes and shapes , including finely crystalline anhedral blocky spar, fine to coarse bladed druse, and large single crystals which take on the shape of an intergranular pore. In ad dition, echinoderm fragments take on large syntaxial overgrowths which oc clude some pore space. Finer-grained layers and more tightly packed -
1 64
Eolian
Figure 35-Photomicrograph showing root-h ai r sheaths extending from calcified rootlet. Smal ler tubules appear to be nonseptate extensions of epider ma l cells (root hai rs) of the larger . m ulticellar tube (rootlet). Pleistocene eol ianite frorn Isla Cancu n , Yucatan P..eninsu la. Plane lig 0 t . Bar scale is 200,t.
patches tend to have a higher percen tage of occluded pores. In a middle Pleistocene eolianite of Israel, pore space is entirely occluded by drusy calcite cement (Gavish and Friedman , 1969).
root-hair and rootlet penetration of Root-hair sheaths Minute tubules of microcrystalline calcite are the carbonate grains. abundant in interstitial pores in some Many grains in association with eolianites (Figs. 34, 35, 36) . These rhizoconcretions and root-hair hollow sheaths are 5 to 1 5 JLm in sheaths also contain long, straight diameter with a wall thickness of 1 to fungal borings 1 to 2 m in diameter . 2 JLm. They are particularly abundant Fungal hyphae grow in close associa Diagenetic Fabrics Related to Roots near rhizocretions and are probably tion with root systems in dunes In some eolianites the cement types calcified replicas of root hairs (Ward , (Webley et al, 1 952) and are filamen and other early-diagenetic features are 1 975). Most sheaths are composed of tous plants that can exist in the finely crystalline calcite similar to the shallow subsurface. associated with the root systems of dune plants. For example in Yucatan , microcrystalline-rind cement (Fig. 34) Subaerial crusts Caliche or and some are partly made up of nee eolianite needle-fiber cement and calcrete crusts (Figs. 3 8, 39) are developed on and within Quaternary microcrystalline-rind cement are par dle fibers, the long axes of which lie eolianites in several localities ticularly abundant in the vicinity of tangent to the tubules. root traces and in paleosols, but (Johnson, 1 967; Ward, 1970; Read, Microcodium Diagenetic struc sparry-calcite cement is scarce (Ward, tures composed of calcite prisms ar 1 974; Semeniuk and Meagher, 198 1 ) . 1 975). Other typical diagenetic fabrics ranged in spherical, elliptical, or Criteria for the recognition o f result from precipitation of calcite sheet-like bodies (Fig. 37) are formed subaerial crusts are given i n Chapter around roots and root hairs. 1 of this volume. in some Pleistocene eolianites (Ward, Rhizocretions Rhizocretions or 1 975; Calvert et al , 1 975) . These root-casts (Fig. 33) may be recognized structures , which are widely reported Progressive Early Diagenesis in cores as vertical or horizontal from limestone and caliche, are refer The progressive development of tubular structures of hard brown red to as Microcodium. Klappa ( 1 978) early-diagenetic fabric in eolianites microcrystalline calcite. They range in found evidence that they are the pro has been studied in Bermuda (Land et diameter from a few millimeters to duce of calcification of mycorrhizal al , 1 967), Israel (Gavish and Fried several centimeters. Generally the in (root plus fungus) associations. man, 1 969), and Yucatan (Ward, ternal structure consists of wavy Microscopic borings Grains near 1 970, 1 975). rhizoconcretions may be riddled with laminae of microcrystalline calcite Most of the Holocene dunes in tubular borings 1 5 to 2 5 m in (caliche-like), concentric about the Yucatan are at least weakly lithified. diameter (Fig. 36) . The holes are long axis of the tubes. The core of In general, more pore space is oc about the same size as root-hair the rhizocretions may crudely cupied by sparry-calcite cement in preserve some of the cell structure of sheaths (some of which are preserved progressively older Holocene the root. in the holes) and may be the result of eolianite. Cement in young eolianites -
-
-
-
-
Eolian
1 65
Fig u re 36-SEM photomicrograph show i n g mic rocrysta ll ine·calcite root-hai r sheath (center) sur rounded by needle fiber cement. Pleistocene eol ianite, Isla Muje res off the Yucatan Pen i nsula.
is not extensive enough to entirely oc clude porosity, although intergranular porosity may be substantially reduced where the rock is cemented with coarsely crystalline spar. Porosity ranges from about 25 to 40D?o in the Holocene eolianites . Almost no dissolution of grains occurs in these calcarenites. On Bermuda, the youngest Pleistocene eolianite is uncemented or is cemented at grain contacts only; porosity is about 20o/o where cemented. The middle Pleistocene dune rock is commonly cemented with grain-skin cement (microcrystalline-rind) ; porosity is 3 0 to 45% . The oldest Pleistocene eolianite is extensively cemented and aragonite grains are dissolved; porosi ty is 36o/o , the dissolution nearly balanced by cementation.
In Israel recent carbonate dunes are unconsolidated. Upper Pleistocene
eolianite is cemented at grain-to-grain boundaries; much of the interparticle porosity is retained . Quartz and other silicate grains are partly replaced by calcite. In middle Pleistocene
eolianite, the intergranular pore space is entirely occluded by drusy calcite cement . Aragonitic skeletal fragments were dissolved out, forming molds now filled with calcite. The diagenetic fabric of the oldest Pleistocene eolianite is essentially the same as that of the middle Pleistocene dune rock . Pleistocene eolianites o f Yucatan are of at least three slightly different ages, but all are presumably late Pleistocene. The youngest Upper Pleistocene eolianite is cemented predominantly with microcrystalline rind and needle-fiber cement with more sparry-calcite cement, and porosity is slightly less. The oldest of the Yucatan eolianites has more sparry cement and lower porosity (less than 25%) than do the others. Calvert ( 1 979) and Calvert and others ( 1980) found that there are dif ferences in the route and rate of diagenesis in Holocene and Pleistocene eolianites from different parts of Mallorca, depending on the original composition and the climate.
AGES OF CARBONATE EOLIANITES Pleistocene Examples
Most carbonate dunes and eolian limestones are classed, according to age, as either Holocene or Pleistocene. No direct relationship between degree of lithification and age is apparent, although all or nearly all nonlithified, active dunes are of modern or Holocene age - not Pleistocene. Many Holocene dunes, however, are partly cemented . Pleistocene dunes may be either weakly or strongly cemented, but all of them are at least partially lithified (Fig. 1 2) . In many regions with extensive ridges or terraces of stabilized dune sands attributed to Pleistocene age, modern active carbonate dunes are also present. Dunes of these two age groups may occur one above the other (Table 3) as on the Libyan coast west of Tripoli where uncemented Holocene dunes are described as overlying lithified Pleistocene dunes (Glennie, 1 970),
1 66
Eolian
Figure 37-Photomicrograph showing Microcodium (glob u l a r bodies with
radiating prisms and hollow centers) in Pleistocene eol i a n it e , I s l a Contoy, off the Yucatan Pen i nsula. Crossed n i co l s. Bar scale is 20011.
and in Australia where loose Holocene dunes are on top of the consolidated dune rocks of the Coastal Limestone (Fairbridge , 1950). In most regions of eolian carbonate sands, including western India, the Yucatan coast, and elsewhere, rem nants of early dune limestones are located at higher levels or in different positions from the active coastal dunes of today. Bermuda, moreover , is one of the few localities of abun dant Pleistocene eolianites in which modern carbonate dunes are uncom mon . This lack of active dunes can be attributed to the islands which are now "heavily wooded and quite free from drifting sand" (Sayles , 1 93 1 ) . A partly submerged cliff of cross stratified eolian limestone at Harring ton Sound, Bermuda, is illustrated by Verrill ( 1 907). Generalized age designations such as "early Pleistocene, " " late Pleistocene, " "sub-Recent , " "early Holocene, " and "late Quaternary, " are applied b y various geologists in describing the carbonate eolianites of certain regions . Determinations based on radio-carbon dating have only been obtained in some places. Thirty six age determinations of late Pleistocene and of Holocene eolian sands made in the Abu Dhabi area of the Persian Gulf (Evans et al, 1 969)
with the radio-carbon method were Ancient Eolianites used in interpreting events occurring Relatively few carbonate rocks over the last 7,000 years, including from the pre-Pleistocene have been the eolian sand-dominated late interpreted as eolian limestones. A Pleistocene and a later time of rising principal reason for this scarcity is sea and mixing of sand with suba the great difficulty in recognizing queous carbonates. wind-blown carbonate deposits and in The necessity of carefully selecting differentiating between them and samples of carbonate sand for dating other carbonate sands of the near is illustrated by the discovery of late shore environment . Dune composition Pleistocene ooids in modern sands of and dune texture are often essentially coastal terraces in southern Tunisia. the same as those of adjacent beaches This modern oolitic sand, which was or bars, as one probably was derived dated at 20,000 to 30,000 years before from the other. This feature is strik the present by carbon- 1 4 measure ingly illustrated in Bermuda ment, has been shown (Fabricius, (MacKenzie, 1 964b), along shores of 1 970) to represent reworked material ; Bahama Islands (Ball, 1967), on the ooids of that type are not forming to Mediterranean coast of Libya (Glen day in the region. nie, 1970) and in many other places . Pleistocene eolian limestones in One of the oldest carbonate rocks regions of extensive source sediment to be attributed to an eolian origin is have accounted for some tremendous the upper Precambrian Wilhite For accumulations. In Bermuda, for in mation of the Ocoee Supergroup in stance, 950Jo of exposed land is com eastern Tennessee (Hanselman et al , posed of this rock (MacKenzie 1 974). This formation is said to con 1 964b). In some regions, dunes of tain an appreciable number of car Pleistocene age are now submerged, bonate rocks that "compare presumably due to changes in sea favorably with modern shallow-water level. A notable example is at Shark [types]" and are interpreted to in clude subaerial deposits. Elsewhere Bay, Australia, where "dunes which these carbonate deposits are described were formed during the low stand of the sea level now separate Shark Bay (Hanselman et al, 1 974) as having from the Indian Ocean" (Malek evolved partly in a subaerial environ ment. Although many of their Aslani, 1 973) . sedimentary textures have been
Eolian
1 67
Figure 38-Cal iche layers developed on Pleistocene eol ianite, Isla Muje res off the Yucatan Peninsula. Micrit ized eolianite is ove r la i n by wavy, lami nated crust. which is overlain by conglomeratic layer with crude reverse-g raded bedding. Photo by W. C. Ward .
destroyed by diagenetic recrystalliza tion, enough remain "to permit the identification of their depositional en vironments . ' ' A few records from the Permian period have been interpreted as wind deposited carbonate rocks. These records include the Magnesian Limestone from Scarcliffe, Der byshire, England , described by Glen nie ( 1 970) as possibly "originally deposited as a carbonate dune on the desert shores of the Zechstein Sea, and was later dolomitized . " The Great Oolite Series o f the Jurassic age in England was con sidered, many years ago (Evan s , 1 900) t o b e of eolian origin because cross-bedding and microscope sec tions showed a close resemblance to eolian limestones of Kathiawar, In dia, and to West Indian eolianites. Subsequent work, however, suggests that these rocks are not eolian , but probably represent subtidal bars and shoals (Klein, 1 96 5 ) . The strata that
contain reptilian eggs, overlying the oolite beds, were likewise believed to be of eolian origin (Evans, 1 900). Jurassic
ACKNOWLEDGMENTS For assistance in compiling the extensive worldwide data and in preparing a syn thesis of this material , appreciation is ex tended to Sarah Andrews. For drafting the several figures included, thanks are given to Willi am Chesser and for much constructive criticism to T. S. Ahlbrandt.
SELECTED REFERENCES Ahlbrandt, T. S . , 1 975, Comparison of textures and structures to distinguish eolian environments, Killpecker dune field, Wyoming: Mtn. Geologist, v. 12, no. 2, p . 61-63. __ S. Andrews, and D. T . Gwynne, 1978, Bioturbation in eolian depo sit s : Jour. Sed. Petrology, v. 48, no. 3, P ·. 839-848. Anderson, G. A . , 1950, 1 940 E. W. Scripps cruise to the Gulf of California, Pt . I, Geology of islands and neighbor ing land areas : Geol . Soc. America M em . 43, 53 p . Auden, J . B . , 1952, Some geological and
chemical aspects of the Rajasthan salt
problem : Proc. Symp. Rajasthan
Desert , Nat! . lnst . Science India Bull . , v . I , p . 53-67. Bagnold, R. A . , 1943, The physics of blown sand and desert dunes: New York, Will iam Morrow and Co . , 265 p .
Ball, M. M . , 1 967, Carbonate sand bodies of Florida and the Bahamas: Jour. Sed. Petrology, v. 37, p. 556-591 . Becher, J. W . , and C. H . Moore, 1 976, The Walker Creek Field; a Smackover diagenetic trap: Trans. , Gulf Coast Assoc. Geol . Socs . , v. 26, p. 34-56. Bernard, H . A . , and C. F. Major, Jr . , 1963, Recent Meander Belt deposits of the Brazos River; an alluvial "sand" model: AAPG Bull . , v. 47, p. 350. Bigarella, J. J . , R. D. Becker, and G. M . Duarte, 1 969, Coastal dune structures from Parana [Brazil] : Marine Geology, v. 7, no. 1 , p. 5-55. Bird, E . C. F . , 1 972, Ancient soils at Diamond Bay, Victoria: Victorian Naturalist , v. 89, no . 12, p . 349-353. Biswas, S. K., 197 1 , The miliolite rocks of Kutch and Kathiawar, western India: Sed. Geology, v. 5, p. 1 47- 1 64. Bramkamp, R. A . , and R. W. Powers, 1958, Classification of Arabian car bonate rocks: Geol . Soc. America Bull . , v. 69, p . 1 305- 1 3 1 8. Bretz, J. H . , 1 960, Bermuda; a partially drowned, late mature, Pleistocene karst: Geol. Soc. America Bull . , v. 7 1 , p . 1729- 1754.
Calvert,
F., 1979, Evolucio diagenetica en els sediments carbonatats del Pleistocene Mallorqui: Univ. Barcelona, Ph.D. thesis, 273 p. __ L . Pomar, and M. Esteban , 1 975, Las rhizocrecioues del Pleistocene de Mallorca: Univ. de Barcelona, Inst .
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Eolian
Fig u re 39-H a rd dense cal iche crust whic h caps ridges of Pleistocene eolianite on Isla M uj e res off the Yucatan Peni nsu la. Caliche crust helps preserve the depositional topography of the dune rock. Also it i s an aquiclude that pro tects u nde rlying eolianite from the modern c l i mate , thereby inh i bit ing diagenesis and preserving porosity and permeability. Photo by W. C . Ward.
Invest. Geology, v . 30, p. 35-60. F. Plana, and A. Traveria, 1980, La tendencia mineralogica de las eolianites del Pleistoceno de Mallorca, mediante Ia aplicacion del metodo de Chung: Acta Geologica Hispanica, v. 15, p . 39-44. Carter, H. J . , 1 849, On foraminifera, their organization and their existence in a fossilized state in Arabia, Sindh, Kutch , and Khattyawar : Jour. Bombay Branch , Royal Asiatic Soc . , v. 3, pt. 1 , p . 158. Chapman, F . , 1 900 , Mechanically-formed limestones from Junagash (Kathiawar) and other localities: Geol. Soc. Lon don Quart . Jour . , v. 56, p. 559-583, 588-589. Chimene, C. A., 1976, Upper Smackover Reservoirs, Walker Creek Field area, Lafayette and Columbia Counties, ___
Arkansas, in North American oil and gas fields: AAPG Mem. 24, p. 1 77-204. Cloud, P. E . , Jr . , 1 952, Preliminary re port on the geology and marine en vironments of Onotoa Atoll, Gilbert Islands: Pacific Science Board, Nat!. Research Council , Atoll Research Bull . ,
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Hopkins Univ. Studies in Geology, no . 19, p. 1 97-300. Emery, K. 0 . , 1956, Sediments and water of Persian Gulf: AAPG Bull . , v. 40, p. 2354-2383. Erwin, C . E . , D . E. Eby, and V. S . Whitesides, 1979, elasticity index; a key to correlating depositional and diagenetic environments of Smackover reservoirs, Oaks Field, Claiborne Parish , Louisiana: Trans. , Gulf Coast Assoc. Geol. Socs . , v. 29, p. 52-62. Evans, G. V., P. B. Schmidt , and H. Nel son, 1 969, Stratigraphy and geologic history of the Sabkha, Abu Dhabi, Per sian Gulf: Sedimentology, v. 1 2 , p . 145- 1 59. Evans, J . W . , 1900, Mechanically-formed limestones from Junagarh (Kathiawar) and other localities: Geol . Soc. London Quart . Jour . , v. 56, p . 559-583, 588-589. Fabricius, F. H . , 1 970, Early Holocene ooids in modern littoral sands reworked from a coastal terrace, southern Tunisia: Science, v. 1 69, p. 757-760. Fairbridge, R. W . , 1950, The geology and geomorphology of Point Peron,
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86, p. 68-87. Folk , R. L . , 1 967, Carbonate sediments of Isla Mujeres, Quintana Roo, Mex ico, and vicinity, in A. E. Weidie, ed . , 2d ed . , Field trip to peninsula of Yucatan guidebook : New Orleans Geol. Soc. , p. 1 00- 123 . ___ M. 0. Hayes, and R. Shoji, 1 962, Carbonate sediments of Isla Mujeres, Quintana Roo , Mexico and vicinity, in Yucatan field trip guidebook: New Orleans Geol . Soc . , p. 85- 1 00. Fosberg, F. R . , 1 953, Vegetation of cen tral Pacific atolls, a brief summary: Pacific Science Board, Nat!. Research Council , Atoll Research Bull. , no . 23, p . 1·26. Frazier , D . E., and A. Osanick , 1 96 1 , Point-bar deposits, Old River locksite, Louisiana: Trans. , Gulf Coast Assoc. Geol . Socs., v. l l , p. 1 21 - 1 37. Gavish, E . , and G. M. Friedman , 1 969, Progressive diagenesis in Quaternary to late Tertiary carbonate sediments; se quence and time scale: Jour . Sed. Petrology, v. 39, p. 980- 1006. Glennie, K . W . , 1970, Desert sedimen tary environments, in Development in
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plants and plant-root structures associated with aeolian sand: Palaeogeography, Palaeoclimatology, Palaeoecology, v . 23, p. 77-87. Goudie, A. S . , B. Allchin, and K. T. M .
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Hedge, 1973 , The former extensions of the Great India Sand Desert: Geog. Jour . , v. 1 39, pt. 2, p. 243-257. and C. H . B . Sperling, 1 977, Long
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distance transport of foraminiferal tests by wind in the Thar Desert, northwest India: Jour. Sed. Petrology, v. 47, no. 2, p . 630-633.
nites and paleowinds: Sedimentology, v. 3, no . 1 , p. 52-64. Malek-Aslani, M . , 1 973 , Environmental modeling; a useful exploration tool in
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Graf, D. L. , 1960, Geochemistry of car bonate sediments and sedimentary car bonate rocks; part I , carbonate mineralogy and carbonate sediments : Ill. State Geol. Survey, Circ. 297, 39 p . Hanselman, D. H . , J . R . Conolly, and J. C. Horne, 1 974, Carbonate en vironments in the Wilhite Formation of central eastern Tennessee: Geol. Soc. America Bull . , v. 85, p. 45-50. Harms, J. C., P. W. Choquette, and M. J. Brady, 1 974, Carbonate sand waves, Isla Mujeres, Yucatan, in Fi eld seminar on water and carbonate rocks of the Yucatan Peninsula, Mexico ; northeastern coast: Ann. Mtg. Field Trip Guidebook, Geol. Soc . America, p. 1 22- 147. Hoque, M., 1 975, An analysis of cross stratification of Gargaresh calcarenite (Tripoli, Libya) and Pleistocene paleowinds: Geol. Mag . , v. 1 1 2, no. 4, p. 393-401 . Illing, L. V . , 1 954, Bahaman calcareous sands: AAPG Bull., v. 38, p . 1-95. James, N . P . , 1 972, Holocene and Pleis tocene calcareous crusts (caliche) pro files; criteria for subaerial exposure: Jour. Sed. Petrology, v. 42, p . 8 1 7-836. Johnson, D. L . , 1 967, Caliche on the channel islands: Mineral Inf. Service, Calif. Div. Mines and Geology, v. 20, p. 1 5 1 - 1 58. Jones, D . J., 1953, Gypsum-oolite dunes, Great Salt Lake Desert, Utah : AAPG Bull . , v . 37, p. 2530-2538 .
Kindle, E . M . , 1 9 1 7 , Recent and fossil ripple mark: Canada Geol. Survey Museum Bull . 25, 1 2 1 p . __, 1 923, Range and distribution of certain types of Canadian Pleistocene concretions : Geol . Soc. America Bull . , v . 34, p . 609-648 . Klappa, C. F . , 1 978, Biolithogenesis of Microdocium ; elucidation: Sedimen tology, v. 25, p . 489-522. Klein, G. deVries, 1 965 , Dynamic signif icance of primary structures in the Mid dle Jurassic Great Oolite Series, southern England: SEPM Spec. Pub.
1 2, p. 1 73-19 1 . Land, L. S . , F . T . MacKenzie, and S . J . Gould, 1%7, Pleistocene history of Bermuda: Geol. Soc. America Bull . , v . 78, p . 993-1006. Lele, V . S., 1973 , The miliolite limestone of Saurashtra, western India: Sed. Geology, v. 10, p. 301 - 3 1 0. MacKenzie, F. T., 1 964a, Geometry of
1 69
Sayles, R. W . , 1 93 1 , Bermuda during the Ice Age: Proc . , Am . Acad. Arts Sciences, v. 66, p. 382-467 . Selim, A. A . , 1 974, Origin and lithifica tion of the Pleistocene carbonates of the Saturn area, western coastal plain of Egypt: Jour. Sed . Petrology, no . 44, p. 757-760.
carbonates: Trans. , Gulf Coast Assoc. Geol. Soc s . , v. 23, p. 239-244. McKee, E. D . , 1 945, Small-scale struc tures in the Coconino Sandstone of northern Arizona: Jour. Geology, v . 5 3 , no . 5 , p . 3 1 3-325 . ___ , 1 966, Structures of dunes at
Semeniuk, V . , and T. D. Meagher, 1 98 1 , Calcrete i n Quaternary coastal dunes in southwestern Australia; a capillary-rise
White Sands National Monument , New Mexico (and a comparison with struc t ures of dunes from other selected areas): Sedimentology, v. 7, 69 p . __, ed . , 1 979, A study of global sand seas: U . S . Geol. Survey Prof. Paper 1052, 429 p .
during the protohistorical and historical periods: Proc. , Rome Symposium, UNESCO and World Meteorol . Organ . , p . 443-450. Sherman, G. D . , and H. Ikawa, 1 958, Calcareous concretions and sheets in soils near South Point, Hawaii: Pacific Science, v. 12, p. 255-257. Shrivastava, P . K . , 1 968, Petrography and origin of Miliolite Limestone of western Saurashtra coast: Jour. Geol . Soc. India, v. 9, no. 1 , p. 88-96. Singh, S . , 1 977, Geomorphological inves tigation of the Rajasthan Desert : Jodhpur, Central Arid Zone Research lnst . , Mono . 7, 44 p . Skocek, V . , and A . A . Saadallah , 1 972, Grain-size distribution, carbonate con tent and heavy minerals in eolian sands, Southern Desert, Iraq : Sed. Geology, v . 8, p . 29-46. Smith , N. D . , 1970, The braided stream depositional environment; comparison of the Platte River with some Silurian clastic rocks, north-central Ap palachians: Geol. Soc. America Bull . , v . 8 1 , p . 2995-3014. Supko, P . R . , 1 97 1 , " Whisker" crystal cement in a Bahamian rock, in 0. P . Bricker, ed . , Carbonate cements : Johns Hopkins Univ. Stud. in Geology, no. 1 9 , p. 143-146. Teichert, C., 1 947, Contributions to the geology of Houtman's Abrolhos, Western Australia: Proc . , Linnean Soc. South Wales, v. 7 1 , p. 1 45- 1 96.
and J . J . Bigarella, 1 972, Defor mational structures in Brazilian coastal dunes: Jour. Sed. Petrology, v. 42, no. 3 , p . 670-68 1 .
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J . R . Douglass, and S . Ritten house, 1 97 1 , Deformation of lee-side laminae in eolian dunes: Geol . Soc. America Bull . , v. 82, p. 359-378. __ and R . J. Moiola, 1 975, Geometry and growth of the White Sands dune field, New Mexico : U . S . Geol. Survey Jour. Research , v. 3, no. I, p. 59-66. Newell, N. D . , and D. W. Boyd, 1 95 5 , Extraordinarily coarse eolian sand of the lea Desert, Peru: Jour. Sed. Petrology, v. 25, no . 3, p. 226-228. __ and J. K. Rigby, 1 957, Geological studies on the Great Bahama Bank, in Le Blank and Breeding, eds . , Regional aspects of carbonate deposition-a symposium: SEPM Spec. Pub . , no. 5 , p . 1 5-72. Phleger, F. B . , and G . C. Ewing, 1 962, Sedimentology and oceanography of coastal lagoons in Baja California, Mexico: Geol . Soc. America Bull . , v. 73, no. 2, p . 1 45- 1 82 . Qadri , S . M. A. , 1 957, Wind erosion and its control in Thar; symposium on soil __
erosion and its control in the arid and semiarid zones: Karachi, Joint auspices of F.A.C.P. and U. N . E.S.C.O . , p. 1 69- 1 73. Read, I. F., 1 974, Calcrete deposits and Quaternary sediments, Edel Province, Shark Bay, Western Australia, in B. W .
Logan , et al, eds., Evolution and
diagenesis of Quaternary carbonate se
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Seth, S. K . , 1 963, A review of evidence concerning changes of climate in India
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Eolian
pany, 53 1 p . Ward, W. C . , 1 970, Diagenesis o f Quater nary eolianites of N.E. Quintana Roo, Mexico: Houston, Rice Univ . , Ph . D . dissert . , 206 p . ___ , 1973, Influence o f climate on the early diagenesis of carbonate eolianites: Geology, v. I, p. 1 7 1 - 1 74. __ , 1975, Petrology and diagenesis of carbonate eolianites of northwestern Yucatan Peninsula, Mexico, in K. F. Wantland and W. C. Pusey I I I , eds . , Belize Shelf carbonate sediment s , clastic
sediment s , and ecology: AAPG Stud. in Geology , no . 2, p. 500-57 1 . ___ 1 978, Indicators o f climate i n car bonate dune rocks, in W. C. Ward and A . E. Weidie, eds . , Geology and hydrogeology of northeastern Yucatan : New Orleans Geol. Soc . , p. 1 9 1 -208. __ and M. J. Brady, 1 97 3 , High energy carbonates on the inner shelf, northeastern Yucatan Peninsula, Mex ico : Tran s . , Gulf Coast Assoc . Geol. Socs . , v. 23, p . 226-23 8 . Webley, D . M . , D . J. Eastwood, and
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1 72
lidal Flat
Fig ure 1 -Schematic d iagram showing sedimentary environments, their rel at ion ship to sea level and relative a m ounts of capillary evaporat i o n . Facies changes are tran siti o n al with the change from supratidal to continental ( l a nd) being the most gradual and subtle of the t h ree.
Figu re 2-Biock diagram schematically showing major facies on the Andros I sland onlap (transgressive) tidal flat model. Supratidal zones are shown in brown, the major one being the supratidal marsh wh ich, because of onlap sedimenta t ion, underl ies the entire tidal flat syste m , except where removed by tidal channel e rosion.
R
ecognition of ancient tidal flats1 in the geologic record, whether they be siliciclastic or carbonate, owes much to sedimentary and diagenetic investigations of modern analogues. The classic studies o f northern European siliciclastic tidal flats by Van Straaten ( 1 954), those of the English Wash area by Evans ( 1 965), and finally the sophisticated refinements of the various sedimen tary processes of this special environ ment by Reineck ( 1967) opened an important era of understanding. Ad ditional refinement of clastic tidal flat processes was provided by Klein ( 1 97 1 , 1 972) . The extension of ideas
and concepts developed from siliciclastic investigations to areas of ancient carbonate tidal flat deposition soon furthered geologic understan ding of these rocks. It was the recognition of similarities and dif ferences between modern siliciclastic tidal flats and ancient carbonate counterparts that stimulated interest in modern carbonate tidal flats. Workers in carbonate counterparts owe much to the early investigations on modern stromatolites by Black (1933), whose work preceded realiza tion of the economic importance of tidal flats. Later studies of modern sediments on the Bahama Banks by Illing ( 1954) and Newell and Rigby
( 1 957) helped focus attention on the Andros Island tidal flats; however, it was two "turning point" discoveries that gave tidal flats worldwide in terest and more sharply focused the investigations that followed. The first discovery was that dolomite is form ing on modern carbonate tidal flats and in areas where physical and chemical conditions are similar to those of large tidal flats; for example, Netherlands Antilles (Deffeyes et a!, 1 965); Persian Gulf (Wells, 1 962); Bahamas (Shinn et al, 1965), and the Qatar Peninsula of the Persian Gulf (llling et al , 1 965). Until then, all forms o f dolomite were considered an enigma, because there were no
lidal Flat
1 73
Figure 3-Biock diagram schematically showing major facies on the Persian Gulf Trucial Coast offlap (reg ressive) tidal flat model. Major supratidal zone is the sabkha, the arid climate equivalent of the supratidal marsh. Supratidal sabkha sediments have accreted over the gypsum, algal mat, burrowed inter tid al and subtidal lagoonal sediments. In many places a cemented crust which forms in the upper subtidal or lower in tertidal zone has been incorporated within the regressive sequence. The gyp sum zone is laterally equivalent to and can be traced into a continuous anhydrite layer farther i n land beneath the sabkha. Tidal deltas composed main ly of ooids form at tidal passes through the c ross-bedded barrier island sands. The barrier island is composed of cerithid gastropod sands and ooids with scattered coral f ragments . Coral reefs grow just seaward of the islands but are absent opposite the tidal deltas. Much of the tidal delta ooids has been cemented
under submarine conditions, especial ly on the seaward side, and beachrock is abundant in the barrier island sands. Composition of facies underlying the ba r-
rier island, coral reefs and tidal deltas is not well known; however, a thin sub marine cemented c rust forms the substrate control ling coral g rowth.
modern analogues . The second discovery was the recognition and documentation of economic oil reserves in Paleozoic carbonate tidal flat accumulations by Roehl ( 1 967). That study paved the way for recognition of many ancient oil producing tidal flats. Owing to the above breakthroughs, detailed investigations of both diagenetic and sedimentary processes proceeded rapidly. Studies were con ducted on the Persian Gulf Trucial Coast by Kendall and Skipwith ( 1 969), Kinsman ( 1 964), Butler ( 1 965), Evans ( 1 966), Evans and others ( 1 969), and Schneider (1975), and by the Royal Dutch Shell group on both the Trucial Coast and Qatar Peninsula (Purser and Evans, 1 973; Shinn, 1 973a). Additional studies in similar climatically arid settings in western Australia include those of Logan and others ( 1 964), and Hagan and Logan ( 1 975). Humid-climate tidal flats were studied at Andros Island in the Bahamas by Shinn and others ( 1 969), followed eight years later by a strikingly illustrated book by Hardie ( 1 977). Studies of arid en vironments in the Persian Gulf by the Royal Dutch Shell group and others were compiled in a comprehensive book edited by Purser ( 1 973). The most recent compilation on tidal
flats, which includes both modern and ancient siliciclastics and car bonates, is a casebook edited by Ginsburg ( 1 975). For many years , geologists recognized mudcracks in the geologic record and realized that their presence suggested sedimentation at or just above sea leveL Now, with more sophisticated knowledge, we know of other diagnostic indicators, and together with information from modern examples, we can determine paleogeographic settings , geometries, and stratigraphic traps associated with these ancient deposits. For ex ample, tidal flats generally form belts parallel to land and may be bordered by open marine or basinal deposits that can serve as petroleum source areas. We also know that the various facies within a tidal flat system, such as intertidal, supratidal and channel deposits, can form reservoirs and seals with predictable distributions. The key to using this knowledge, however, is, as with other sedimen tary rocks, accurate identification of the various sedimentary structures in dicative of tidal flat accumulation . The purpose of this chapter is to generalize our vast but diffuse knowledge of the various kinds of tidal flats, both modern and ancient, and to graphically describe the most
diagnostic sedimentary structures and sequences . Modern examples will be used frequently throughout this chapter, because they provide the fundamental basis of understanding. In addition, attempts will be made to speculate and pictorially show how stratigraphic traps may be produced in various parts of a tidal flat system. 2 TIDAL FLAT ENVIRONMENTS
AND SEDIMENTATION
Tidal flats are integrated systems . All tidal flat systems, except those in areas dominated by wind tides, are composed of three basic environ ments: supratidal, intertidal, and sub tidal . Within these environments there are several subenvironments . The basic environmental zones shown in Figure 1 include the following: (1) supratidal sediments , those deposited above normal or mean high tide and exposed to subaerial conditions most of the time because they are flooded only by spring and storm tides; spring tides occur twice each month, and 'Tidal flats and their arid counterparts, called sabkhas, are interchangeable in this text. 2Highlights of this chapter are depicted in the film, "Stratigraphic Traps: the Tidal Flat Model ," produced by AAPG.
174
lidal Flat
Fig u re 4A-Siab of p last ic-impregn ated core of supratidal l eve e laminations showing a 2-cm -th ick lens of storm deposited flat pebble breccia at top of core. Arrow shows i m bricate lami nated pebbl e . Note b i rd ' s-eye voids scattered here a n d there, which are distinct from plant roots. Also n oti c e tran sition to less Figure 48-Thin section showing g raded well laminated upper i nte rtidal sediment pellets and lateral disconformity of supratidal levee laminations. Material at base of core. O u t l i n ed a rea shows location of thin section shown in Figu r e removed from ci rcle was analyzed by X·ray diffraction . Sediment contains ap 48. p r oxi ma tely 5 07o modern dolomite. Small circ les show bird's-eye voids which parallel laminations.
storm tides, the largest of all , occur sporadically during certain seasons and are less frequent; (2) intertidal sediments, which lie within normal high tide and normal low tide; they are exposed either once or twice daily depending on the tidal regime and local wind conditions; and (3) sub tidal sediments, which are seldom, if ever, exposed to air. For the purpose of this paper, the term "subtidal" is restricted to those sediments seaward of a tidal flat system or within a system, such as in tidal channels. The information presented in this chapter is based on two models, the humid, transgressive or onlap Andros Island model (Fig. 2) and the arid, regressive or offlap Persian Gulf model (Fig . 3) . In both models the major supratidal flat (the supratidal marsh at Andros Island and the "sabkha" of the Persian Gulf), forms a belt paralleling the land. This supratidal belt invariably merges tran sitionally with continental deposits above the reach of even the highest storm tides. Minor supratidal en vironments occur as levees on the outer bends of meandering tidal chan nels within the intertidal zone, or on
the back side of low-lying beach ridges (Fig. 2) . The important supratidal zone, the one that is most extensive and most likely to be preserved, is the supratidal marsh or its arid equivalent , the sabkha. Because of the mechanics of supratidal sedimentation, there is a basic difference between sedimentary structures formed on levees and beach ridges'and those deposited in the marsh or sabkha (Hardie, 1 977). Levees and beach ridges are flooded frequently by spring tides that carry little sediment; thus, individual sedimentary laminae are thin (Fig. 4) . Storm tides, even though they may carry large sediment loads, tend to deposit little in these environments, because soft , pelleted sediment does not easily settle out when water moves rapidly over these en vironments. In the supratidal marsh, however, highly sediment-charged water eventually comes to rest and layers exceeding 2 em in thickness may be deposited in a few hours (Fig. 5; Shinn et al, 1 969) . During storms,
large quantities of sediment settle on intertidal and subtidal flats, but few laminations survive in the subtidal zone due to bioturbation. Some are preserved, however, in the intertidal zone of arid tidal flats because of ex treme salinities. Storm layers in the supratidal marsh are almost invariably sand wiched between layers of organic carbon-rich algal material , which pro liferates during the long intervals be tween storms. The individual algal or storm layers can be traced laterally for tens of meters. Such thick algal material is generally lacking on levee and beach ridge environments, and individual thin laminations can seldom be traced laterally for more than a few centimeters (Fig. 4; Shinn et al, 1 969). SUPRATIDAL ZONE DIAGNOSTIC SEDIMENTARY STRUCTURES Mudcracks
Probably no single sedimentary structure is better known or more in dicative of a depositional environ ment than polygonally arranged mud cracks caused by shrinkage of car bonate mud (Figs . 6A, 6B, 7A, 7B) . Although such shrinkage can occur in playa lakes far from the sea, they are
lidal Flat
1 75
Fig u re 5A-Siah of p
F i g u e 58-Po r t ion o f s u p r a tidal marsh c o re ta ke n on a part of the marsh a few ce'ltimeters h igher than that in F i g u r e 5 A This core conta ins lightly ;j thified and desiccated laye r s and g reater oxidation
of organ ic -rich layers t h a n tt1<:! i'1 Fig w e 5A Large objects are mangrove and grass roots .
F ig u re SC Portion of a supratidal rn a r s l i core artit ic ially compacted to a depth equivalent to 900 ft (274 m) ot buri a l . Note :aterai extension of o rganic matter and des: r u c t i o n of bi rd 's-eye and root vo1ds. Gast ropod shells show no sig n s of c r ush i n g . Scale i n e m .
more often associated with the supratidal environment of tidal flats that border the sea. Syneresis cracks, which may look similar to desiccation cracks , are known to form in subtidal clay mineral deposits . The phenome non, however, has not been documented in carbonate sediments. Shrinkage cracks are commonly observed in modern intertidal set tings, but there is little evidence that they are preserved in this environ ment. The considerable information available suggests that mudcracks are best preserved in the supratidal and possibly the upper part of the inter tidal zones. Thick storm layers dry and shrink
to produce large mudcracks and mud polygons, whereas thin layers shrink to form smaller cracks and polygons. When thick storm layers are deposited in areas lacking prolific algal mats, shrinkage cracks tend not to perpetuate themselves upward through subsequent layers . When algal mats are present , however, cracks tend to be widely spaced and perpetuate upward with the addition of each succeeding algal or sedimen tary lamination, even when the layers are thin. Thus, where algae are abun dant, the rule that thick layers result in large desiccation cracks and thin layers make small cracks generally does not hold true. Cracks in sedi-
ment dominated by algae may be very large (Figs. SA, 8B) or small (Figs. 9A, 9B) or nonexistent (Fig. lOA). The reason for this difference is not understood but is probably related to the length of exposure (the exposure index of Hardie, 1 977) and the thickness of individual storm layers within the mats. Intertidal flats are often devoid of algal mats or shrinkage cracks due to the activities of organisms (Fig. lOB). The classical mudcrack is V-shaped. All mudcracks initially form in this shape, but in carbonates the "V" shape often changes. During exposure, the edges of the cracked mud polygons become rounded
1 76
1idal Flat
Figure 6A-Depositionai surface of supral"dal l i me mud on Ciuett Key i n Florida Bay showi ng healed mudc racks in a layer deposited by H u r r icane Donna in 1 960. Cracks are lilred with porous and permeable materia! weath ered from adjacent polygons and sediment deposited during subsequent storm floodlngs.
Figure 6 8-Mudcracks of Ordovici a n age i n Kentucky. N ote simila rity with rnoc.le: n examJJ!e in Figure 6A. Pt1oto courtesy of Earl Cressman (U .S.
Geological Survey).
through weathering. The result can be a feature that is sausage-shaped in section , like the well known boudinage of ancient limestones (Figs. 1 1 , 1 2). Because of this weathering, dense, impermeable mud polygons can become encased in the more porous and permeable sediment derived from the weathering process. An important aspect of this process is selective dolomitization. Shinn
( 1 968a) reported on selective dolomitization of permeable sediment surrounding lime mud polygons in the lower Florida Keys (Fig. 1 3). This form of early selective dolomitization is believed to be responsible for many limestone layers, lenses, and flat peb bles in ancient dolomite. These features are especially common in tidal flats of Paleozoic age (Fig . 1 2).
Laminations
Laminations in tidal flat sediments generally result from spring or storm tide deposition (Ball et al, 1 963). As suggested earlier, they tend to be thicker in the more distant or land ward portions of the system (the supratidal marsh or the sabkha; see Figs. 2, 3 , 5A, 5B, 5C) and thinner in the more seaward part (Figs. 4A, 4B), although combinations of thick and
Tidal Flat
1 77
F i g u re 7A-Aigae selectively growing on more porous and permeable sed iment filling mudc racks in sediment depositea by H urricane Donna in 1 960 (Sugarloaf Key, Florida). The mud polygons are less porous and permeable than sediment in c racks aM therefore less susceptible to doiumitlzation: �ee Flgwe 1 3 .
Figure 78-Sh rinkage cracks in algal mat. whictl is not related to storm layer, Andros Island tidal fiat. Compa re wi!h exa r nple in Figure ?A.
thin laminae can occur in any single location. Thick layers in the more landward part of arid sabkhas are often destroyed by desiccation, however . Tidal flat sediment of both modern and ancient age is deposited as layers of sand-sized mud pellets, and although these pellets may be destroyed during dolomitization or compaction, they may even form
micro-crossbedded laminae. Graded pellets are commonly present in supratidal storm layers (Fig . 4B) . Rip pling, a feature restricted to fine- to coarse-grained sizes and generally thought to be a submarine phenomenon, can be common in supratidal facies (Fig. 1 4). Horizontal laminations, whether graded or ungraded, thick or thin, with or without crossbedding , are
restricted to supratidal and upper in tertidal conditions in modern tidal flats as well as to those of late Paleozoic through Cenozoic age. The presence of sucl;l laminations is in directly controlled by burrowing organisms. In older Proterozoic or lower Paleozoic rocks, before the development of extensive infaunas or browsing organisms, sedimentary laminations may have been preserved
178
lidal Flat
Fig u re 8A-Large mudcrack pattern in thick algal mat on edge of brine-fi lled pond at lnagua Island in the Bahamas. Gypsum is precipitating in cracks and on margins of upturned crack edges.
Figure 88-Similar algal mats on tidal flats of the Trucial Coast in the Persian
Gulf.
Figure 9A-Desiccated algal mat in
lower supratidal zone i n the Persian
Gulf. Small mound next to lens cap in upper center of photo is made by algae populati ng pelleted sediment thrown up by burrowing worm. With th e i r convex upward laminations and central canal, these mounds resemble the Precam brian algal featu re, Conophyton (see Shinn. 1 972).
lidal Flat
1 79
Fig u re 98-Siiced section of a l ga l sedi ment shown in Fig u re 9A Note desicca t ion crack (A) covered by win dblown sediment containing 65 % dol omite (B).
Figure 1 OA-Localized algal mat without desiccation c racks in supratidal zone of the Persian Gu lf on the Qatar Peninsula_
in all environments, as suggested by Garrett ( 1 970) and Hofman ( 1 969). Although storms may deposit layers in the subtidal and lower intertidal zones, abundant burrowing organisms that thrive in these environments
quickly churn and homogenize nearly all primary laminations . Few organisms, on the other hand, can en dure the rigors of exposure and fluc tuating salinity in the supratidal en vironment ; thus, there is little
destruction of primary sedimentary features . Plant life is also severely restricted, especially in arid tidal flats or sabkhas; therefore, roots are seldom so abundant as to completely destroy sedimentary structures. Plant
1 80
Tidal Flat
Figure 1 08-Burrowed intertidal sedi ment in the Pe rsia n Gulf. Small bu r rows were made by crabs and larger depres sions by mud skippers. a small fish that stalks the fl at s at low tide.
Figure 1 1 -Mudcracked supratidal dolomite from Cretaceous rocks in west Texas near San Angelo. Note bird 's-eyes. rounded mudcrack polygons and large crack in thick storm layer with vertical tubes made either by worms or esca pi ng gas bubbles. Similar burrow l ike voids appeared in hurricane depos i t e d layers of similar thickness within a few weeks of depos ition_ Such mudcracked dolomites contai n i ng dinosa u r tracks occur in Cre taceo us rocks in central Texas. Specimen
cou rtesy of Clyde Moore (Louisiana State University).
life and resulting root effects, however, do increase where the sedimentary surface is more elevated or in a landward direction , where supratidal sediments merge into con tinental deposits . Such a transition may also be associated with a facies change to a more siliciclastic environ ment. Such a transition from supratidal carbonates to siliciclastic
redbeds is common in tidal flats of Permian age in west Texas and New Mexico . Algal Structures
The environmental significance o f algal mats and domes became ap parent as a result of Black's ( 1 933) work on Andros Island, Bahamas. Black's studies stimulated interest in
and resulted in publications concern ing algal structures, but it was not until the publication of Logan and others ( 1 964) that the description and environmental significance of various forms became simplified enough for recognition and classification by non specialists. Logan and others ( 1964) recognized three basic forms: (1) flat laminated forms (generally referred to
Tidal Flat
181
Fig u re 1 2 -G ray lenses of l i mestone floating in silty d o l om i te o f Cambrian Ordovician age from M a rylan d . The li mestone lenses are thought to be roun d ed mudcrack polygons that escaped early dolomit ization similar to the modern exa mple i n Fig u re 1 3. Spec imen is from cyc l i ca l tidal flat l i mestone and dolomite sequence described by Matter ( 1 967}.
Figure 1 3-Mode rn cemented supratidal crust from Sug arloaf Key, Florida, show ing a storm l aye r (a r row} composed of sun-dr ied pel leted l ime mud. The sur rounding cemented matrix contains up to 250/o high-calcium dolomite. This style of selective supratidal dolomitization is thought to be the origin of the features in Figure 1 2 .
as algal mats), (2) separate hemispherical or club-shaped domes, and (3) laterally-linked hemispherical shapes . Examples of these may be found in tidal flat accumulations ranging in age from Paleozoic to Holocene (see Figs. 15A, l SB, l 6A, 1 6B). Such examples have been described by numerous authors, in cluding Hofman ( 1 969), Hoffman ( 1 967) , and James ( 1 977). Correct interpretation of tidal flat laminations can be complicated by the presence of algae. Some layers are
induced entirely by algae, whereas others have a purely sedimentary origin. The origin of ancient lamina tions, where algae have long since rotted away, can therefore be ex tremely difficult to determine. One must find clues, such as antigravity structures (for example, sediment on the near vertical sides of domes), to determine if algae have assisted deposition . In Figure 17, a core from the Permian San Andres dolomite in west Texas shows oxidized supratidal laminations grading downward into
Figure 1 4-Ripples formed in pelleted supratidal sediment on a channe l levee at A nd ros I sland . Locality is same as for core shown in Figures 4A and 48. Rip· piing caused during storm and spring tide floodings probably explains laterally discontinuous laminae shown in Figu res 4A and B .
1 82
lidal Flat
figure 1 5A- Modern subtidal to in t e r tidal columnar stromatol ites at Haml i n Pool, Western Australia. Note that heads conti r. ue seaward into subtidal t ion s. In a landward direction, the
condi· heads grade into la!era l ly conti nuou s layers on
low supratidal flats assoc iated with evaporitic m in era ls . Pilato courtesy ol A. N . Ghsburg (University of Miami Comparative Sedimen tology laboratory).
Figure 1 5 8-Same locality as Figure 1 5A but showing rippled intert idal car bonate sands bet\N€en cemented algal heads. I ndividual heads have lon g axes aligned peroendicular to shore and to trend of ripple crests Photo cot Jrtesy of A. N . Ginsburg. For a more complete description of a re a , see Logan and others ( 1 964), and Davies (1 970).
gray reduced intertidal or subtidal sediment. The distinctly laminated domal structure, which is clearly algal in origin, strongly suggests that the more horizontal laminations with which it is associated were also of algal origin. Without the presence of this domal structure, it would be dif ficult indeed to determine if the laminations were physical or algal in origin (see Fig. 1 7) . Algal tubules also provide clues to algal origin , but they
are unlikely to be preserved if t he rocks have been dolomitized. Various forms of algal mats on modern tidal flats are shown in Figures 8, 9, and 10. Observation on numerous occasions of algal mats in the Persian Gulf has shown that they act as sticky flypaper to trap windblown dust. Several times a year, northerly winds, called shamahls, cause "west Texas-type" dust storms that transport fine-
grained material across the Gulf from the Zagros Mountains of Iran. Within hours, this dust coats the mats, im parting a rusty-tan color. Within a day or two, algae recolonizes the sur face, thus producing a distinct lamination of detrital material . The thick brown layer in Figure 9B was interpreted to be of eolian origin. If the layer in Figure 9B had been deposited from water, it would have been mainly aragonitic. Instead, it
lidal Flat
1 83
Figure 1 6A-Lateral ly-linked hemi spheroids (LLH classification of Logan et al, 1 964} formed by stromatolitic algae oi Camb ro-Ordovician age i n M aryiar.d. Photo from same loc al i ty as in Figure 1 2.
Fig u re 1 68-False LLH st romatol ites f'om the Florida Keys. This o rganic-rich stromatol ite-li ke e nc rustation is actually a soi lstone cr ust with little i n common wi th true i ntert ida l and supr atidal algal stromatolites . Wit h out knowing the sed i mentary sequence, i! may be i m · poss i b le to d i ffe rentiate such features from genuine algal structures in the geolog ic record. See M u lter and Hoff meister (1 968) and Robbin and Stipp (1 969) for genetic interp retation of soilstone crusts.
was composed of clays, quartz silt, and about 600J'o detrital dolomite. During several dust storms, vaseline coated glass microscope slides were exposed to the wind and the adhered particles were examined petrographically. Typically, such dust contained about 600fo detrital dolomite. The crystals measured about 30�tm across, with quartz silt in the same size range. The remainder of the material was composed of clay minerals. The obvious conclusion is that supratidal algal laminations in arid
areas can result both from alternating flooding and deposition of marine sediment and algal mat growth or alternating eolian deposition and algal growth . Bird's-eye Structures Bird's-eyes, an old American term which was recently redefined and call ed fenestrae by Tebbutt and others ( 1 965), are considered reliable in dicators of supratidal deposition when they occur in predominantly muddy rocks. Bird's-eyes, or fenestrae, are considered identical to
Figure
1 7 -A core from the Permian San Andres dolomite in west Texas showing trans i1ion from oxidized aloal lamin ated suprat idal dol omite wlth lgal dome (A) and bird 's·eye voids filled with anhydrite (Bj and reduced bur�owed in tertidal dolornite below_ Porosity Is com pletely filled with an hydri te. At least 1 8 sim ilar supratidal-intertidal transitions were observed in the core described in Figure 46. SCale in em.
a
1 84
lidal Flat
Fig u re 1 8-Sem i - l ithified modern supratidal c r ust from Sugarloaf Key, Florida , showing abundant planar-type b i rd 's-eye voids (arrows).
Fig u re 1 9-Th in section photo of fil l ed and partially til led bi rd ' s-eye voids i n dolomitic supratidal c rust from Andros Is land. These early filled voids should resist obl iteration d u ring bu rial and com paction . The cement is e ither a ragonite or magnesium calcite.
the "shrinkage pores" or " loferites" of Fischer ( 1 964). These features are small millimeter-sized vugs that form in supratidal sediments as a result of shrinkage and expansion, gas bubble formation, air escape during flooding , or wrinkles in algal mats (Shinn, 1 968b). In ancient limestones , bird's-eye vugs are generally filled with calcite or anhydrite. Often they contain geopetal internal sediment, as in the Ordovician rocks described by Grover and Read ( 1 978). True supratidal bird's-eye structures should not be confused with other calcite filled voids , which they often resem ble. Shinn (1968b) found by ex-
perimentation and examination of hundreds of modern sediment cores from Florida, the Bahamas, and the Persian Gulf, that bird' s-eye vugs can be divided into two distinct types - planar-shaped, generally un connected voids (Fig. 1 8) , which tend to form parallel to individual laminae, and randomly distributed bubble-shaped voids. The latter almost always occur in supratidal sediments lacking sedimentary or algal laminations. Bird's-eye vugs may form in intertidal and even sub tidal sediments, but evidence that they are preserved in these en vironments is lacking. These features
are preserved, however , in the upper part of intertidal sediments and in crease in abundance through the tran sition to supratidal sediments. Supratidal bird's-eye vugs are preserved because they form in an ac tive diagenetic environment, where early lithification is common and voids can be quickly filled with car bonate cement , internal sediment, or evaporites. Figure 19 shows calcite filled bird's-eyes in modern dolomitic crusts from Andros Island. The bird's-eyes in the Permian example in Figure 17 are filled with anhydrite that probably formed penecontem poraneously with deposition . Some
lidal Flat
i.l
'>; L) :� 0 w � f.I !: ;ll: �
0. �
��
z �·'
::l
Fgurtl 20A -A Dlaatic-i mp(e�r�a�oo Gon.'.l from t he suprat:oal z:orl c of a m�od 1sland in F:orkla Bay . N�le birc ' s ·ey·.e vu�a anci veoi!cal wotm c r g<\S 031Scap� stn.Je:lur�s sncwn by <wows.. W'! rt lc::� l void� arl;! bellevM 1c be s lmil�tr to !l'IMe 01 Cretaceous age eho'�vn in Fi g u re 1 � anc those ol Ordovicia.'"l agb! s'lown ; n Figure22.
. igurt>: .2 013--0cmpar,lo:'l core to ; h :>l 2:-Jowrs ir; Fig,J re �OA. �l r l:! has been ar t i fin'all� nDrnpacte�'" obliterated . l'lote ml'.lttlilr.l tt-xtnre developed in i r>w&r portluo1 or �!>r€ whlcn has beer .-,x;;.gge�ated ty CO�<'t:on. Det:lll o� outlined
r-!<,;ure
Fi9ure 20C·- Detail of m. . c:crack� sto rrTI l�yer� 11� compac ed c e re . Origin cf color var iatiOJl S within st.orm layP.r:': Is Mtlr v•clerstooo OJ1 probably represents l oc:al area:;: of redur.tio.� Orgnnic materlfll has been squceze-:1 1aterall y . ::>ut !o'3.siis o.:� ;;nbroken. Sca le in mrr .
1 85
zoe.
1 86
Tidal Flat
Figure 21 - Portion of a compacted core
of modern suprati
layers and storm layers . Note subvertical i r r eyular stylolite-like l i nes stmwn by ar rows. The l i nes resu lt from compaction
verti cal voids that have a thin organic li ni n g Core from Crane Key in Florida Bay has been compacted to equivalent of 1 0,000 ft {3.048 m} of ove-burden. Scale in mm.
of
.
bird's-eye structures may even be formed by the development of supratidal evaporites (Illing, 1 959). Contrary to an earlier conclusion by Shinn ( 1968b), bird's-eye vugs in uncemented sediment can be destroyed during compaction (Figs. 20A, 208, 20C, 21), as shown by re cent compaction experiments (Shinn et a1, 1 980). The presence of bird's-eye vugs and well-preserved pelletal texture in ancient limestones and dolomites is therefore considered evidence of both supratidal deposition and early cementation (Shinn et a!, 1 980). Pseudo-Bird's-eye
or Fenestral Structures Some ancient tidal flat rocks con tain "subvertical tubular fenestrae ," as described by Read ( 1 975) in rocks of Devonian age and by Cressman and Noger ( 1 976) in rocks of Ordovi cian age in Kentucky. Although prob ably not of the same origin as the planar and bubble-like voids, these features are nevertheless common in both modern and ancient supratidal settings. Shinn (in press) has sug gested that features identifiable as burrows or roots should not be in cluded under the bird's-eye or fenestral classification even when they occur with true planar or bubble shaped fenestrae (bird's-eyes or Ioferites) . Burrows and root tubes can form in other environments; thus, they alone may not be reliable in dicators of tidal flat deposition. When they occur with other exposure
features, however, as is shown later, then they can be considered addi tional criteria for tidal flat deposi tion . Under these circumstances, the term "pseudo-bird's-eye," " pseudo fenestra," or "pseudo-loferite" is preferred. Both pseudo- and true bird's-eyes or fenestrae also form in modern muddy subtidal environ ments, but because of compaction , they are not preserved. They are probably rare in ancient subtidal en-
vironments. Note the vertical voids indicated by arrows in Figure 20A. Although common, their origin has never been adequately explained. The author observed that these features formed within a few weeks in thick (up to 10 em) lime mud layers deposited during Hurricane Donna in Florida in 1 960. They measured 1 to 3 nun in diameter and had a dome shaped upper termination. Unlike tubular voids made by burrowing
lidal Flat
1 87
vertical �ubes in storm layer of Ordovici
Fig u re 22-Calcite·fi lled a
tucky. Althoug h often assoc iated with supratidal deposit ion , such features should not be confused wittl true
Earl IU.S . GeoloaicaLSurvev). . .
bi rd ' s-eye voids. Photo courtesy of
Cre�>�>man
.
worms, these voids seldom penetrate to the surface. These features, how ever, superficially resemble both worm burrows and plant root voids , including the subtidal root voids made by the turtle grass, Thalassia. Although these features resemble bur rows and plant root holes, no worms or roots were observed in the voids formed in fresh storm-deposited sedi ment . Figure 22 shows pseudo-bird's-eye voids in Ordovician rocks that look identical to those observed in Hurricane Donna storm layers. A similar example of Creta ceous age is shown in what is clearly a thick storm layer in Figure 1 1 . Notice in Figure 2 1 the dark vertical stylolite-like markings that resulted from compaction of vertical voids shown in Figure 20A.
Intraclasts
All sedimentary features described earlier are subject to erosion and redeposition to produce distinctive sedimentary structures. Intraclasts, probably eroded and redeposited mainly during storms, occur in two major areas - on the supratidal flats, and in subtidal channels, where they become part of the basal channel lag. Tidal channel deposition and its sedimentary products will be dis cussed later. Because hardening and cementation of muddy sediment on tidal flats is commonly restricted to supratidal conditions, this environ ment, therefore, provides most tidal flat intraclasts. Material eroded from muddy subtidal and intertidal en vironments, on the other hand, is generally in the form of grapestone, individual pellets , or mud-sized par ticles and fossils. Furthermore, because most clasts are derived from the supratidal environment, they therefore contain diagnostic sedimen tary features, such as sedimentary
and algal laminations, in addition to bird' s-eye vugs. Mud polygons produced by desicca tion are highly susceptible to erosion and redeposition . Before-and-after observations showed that the flat peb ble conglomerate (Figs. 23A, 2 3B) was deposited almost instantaneously on a small supratidal flat in Florida Bay during Hurricane Donna in 1 960. It was composed of redeposited mud polygons. Similar intraclasts can be seen in the top of the supratidal levee core shown in Figure 4A. In both ex amples, the intraclasts were deposited as sedimentary lenses 1 to 6 em thick and less than l m across . As shown in Figure 24, intraclasts can also occur as redeposited slabs of dolomitic crust. Those in Figure 24 were stranded in imbricate fashion along the shore of an almost abandoned tidal channel west of Abaco Island in the Bahamas. Similar accumulations occur sporadically on the muddy beach ridge shoreline of the Andros Island tidal flats. Several
1 88
Tidal Flat
Fig u re 23A-Desiccalion polygons redeposited on Cluett Key in Florida Bay duri n g H u rricane Donna in 1 960. Core t aken with paint can is shown in F igure
23B.
Figure 24- Dolomitic supratidal crusts eroded and redeposited in imbricate fashion a l ong edge of nearly abandoned tidal channel . Similar breccias are abu n dant in the geological record.
Fig u re 23 8-Piastic -impreg nated core (location shown i n F i gure 23A) showing imbric:atP. mucl !ntrac:l asts deposit€d dur ing H u rricane Don n a . N ote how weather ing has d isrupted earlier storm laye rs and produced soi1-like int raclasts. Th in algal laye r separates Hu rricane Do nna !ithoclasts from earlier storm layer. Note vertical vo i ds in underlying sediment . Bird ' s·eye voids are abundant .
ancient examples of tidal flat intra clasts are shown in Figures 25 and 26. SoU Clasts
A special kind of irregularly shaped intraclast forms where supratidal sediments are elevated high enough above the saline water table to sup port abundant plant life. In modern humid-to-semi-arid environments, the plants are generally grasses and palms. In these areas a combination of factors, such as alternate wetting
Diagnostic sedimentary laminations and structures are generally lacking in both modern and ancient intertidal accumulations. Lack of laminations in this environment is attributed en tirely to homogenization by burrow ing organisms. Aside from a characteristic oxidized color and a generally lower fossil diversity, inter tidal sediments in humid areas are practically indistinguishable from nearby subtidal sediments. They are distinguishable, however , from the subtidal deposits of tidal channels, which dissect intertidal sediments at regular intervals. INTERTIDAL ZONE In the Persian Gulf (Fig . 3), where the climate is arid, the presence of Intertidal sediments are deposited evaporites in the upper part of the in between normal high and low tide tertidal zone precludes the presence of and generally accumulate as a belt most burrowing organisms. The mid seaward of the supratidal flats and landward of the subtidal zone. Many to-lower part of the intertidal zone (tidal range about 2 m) is dominated tidal flats, especially the onlap or transgressive accumulations at Andros by burrowing fiddler crabs and a small fish of the gobie family that Island (Fig. 2), are dissected by a complex system of tidal channels. In burrows and stalks the flats at low tide to catch crabs. As shown in tertidal flats of the offlap or Figure 3, algal-laminated sediments regressive accumulations in the Per form in the upper intertidal zone and sian Gulf contain relatively fewer extend into the overlying supratidal tidal channels. and drying, shrinkage and contrac tion, disruption by roots and associated burrowers (earthworms), cause internal shifting of sediment components. Layers, for example, become disrupted and may appear as though transported by wind or water currents . Well known to pedologists , this soil process is probably an inci pient stage of caliche formation. Ex amples of this form of intraclast for mation are shown from both modern and Permian rocks in Figures 27 and 28 .
lidal Flat
1 89
Fig u re 25 -Limestone flat pebble brec cia in dolomite supratidal laminations (probably algal) of Ordovi cian age i n Kentucky. Photo c ou rt esy of Earl C ressman ( U . S . Geological Su rvey).
Figure 26-Limestone flat pebbles from
a cyclical tidal flat sequence described by Matter ( 1 967) in a matrix of fossil hash of Cambro-O rdivician age in
Maryland. This acc umu lation may repre sent a basal channel deposit.
Figure 2 7-Soi l processes in the upper part of the supratidal zone on Andros I sland produced these clasts by d i s ru p t i ng individual storm layers. Note numerous bird's-eye voids, both bubble like and planar. Scale in mm. Compare t h is plastic-impregnated modern exam ple with the Permian example i n Figure
28 .
190
lidal Flat
F igu re 28-Anhydrite-filled suprat idal "soil clas t " dolomite from t h e Permian San And res dolomite in west Texas. Sub su rface depth in feet is shown on roc k . Scale in m m .
zone. Because of high salinity and rapid burial by accreting supratidal sediments, reduction and buildup of H2S is common. Only the upper part of the intertidal zone in these accre tionary environments shows signs of oxidation . Bird's-eye vugs, generally the bubble-like variety, may occur in the upper intertidal zone in those areas where algal mats are lacking (Fig . 29). If algal mats are present, the vugs are predominantly of the planar variety . Adjacent Marine Sediment
Figu re 29-Transitional intertidal to supratidal facies from a modern tidal flat
on the Qatar Penin sula in the Persian
Gulf (see S h i n n , 1 973a) showing oxi d ize d seci i ment with bird' s-eye voids (in circles) a nd abundant s h e l ls. Ab u nda nce of �t H:: IIs is u n u s ua l and related to local cond itions. Scale in e m .
Except for tidal channels, the sub tidal zone forms a belt seaward of the intertidal zone (Figs. 2 , 3 ) . This zone of generally muddy sediment is vital to the tidal flat system, because it is the source of sediment needed for ac cretionary growth . For the purpose of this paper , the outer limit of the muddy subtidal zone is 5 km from the intertidal zone. Depending on the geographic setting, the subtidal zone may extend seaward, in some cases for hundreds of kilometers, and in cludes accumulations such as ooid bars , coral reefs, "ramps" composed of both cemented and uncemented Fig u r e 30-Gray reduced bu rrow grainstone, and deep basinal deposits. mott l e d subtidal s ed i m ent from the same Generally, however, the muddy adja core as shown in Figure 29. Sediment does not contain blrd's-eye voids. Scale cent marine sediments within 5 km in e m . are in shallow, low-energy conditions and are thus easily stirred into
Tidal Flat
Figure 3 1 -0bli que aerial photo of the Andros 1 idal flat belt showing location of core transect shown in Figure 3 2 . Note iight-co lo red levee sedi ment on outer bend nea( A' boroered by dark alga! mat coating, 1n turn bordered by shmted fed mang rove trees that peter out in subtidaJ pond area. Supratidal mar sh , colored dark by dense algal mats, is sl1own in distance.
suspension and transported to the flats during storm-tide conditions. Subtidal sediments are predomi nantly composed of pelleted muds, although they can vary in grain size, composition and hardness. These muds are almost always homogenized by burrowing and lack primary sedimentary structures . Never exposed to subaerial conditions, these sediments are also reduced and have a
characteristic mottled-gray color (Fig . 30). In some areas , such as in the lagoon seaward of the Persian Gulf Trucial Coast (Fig . 3) and at Qatar (Shinn, 1 973a), winnowed grainstone layers in the upper subtidal or lower intertidal become cemented to form a resistant crust- Such crusts have low permeability; thus, when they become incorporated into an outwardly ac creting tidal flat , fluids can be effec tively retarded from vertical move ment. Equally important , these resis tant crusts often, but not always, pre vent downward cutting of tidal chan nels, thereby restricting channel deposition to the intertidal zone. In addition to these thin, nearshore crusts, there are more thoroughly
1 91
cemented crusts forming in subtidal sediments farther offshore, starting at a depth of about 3 to 4 m (Shinn, 1 969). These deeper and thicker crusts with eroded and bored upper surfaces may also eventually underlie tidal flat sediments in a progradational setting in which the tidal flat builds seaward . Such crusts have been traced for dozens of kilometers seaward to dep ths of 1 5 to 20 m, and their low permeability of only a few millidar cies (Shinn, 1 969) can also effectively impede vertical movement of fluids. The existence of similar layers in an cient tidal flats could have a great in fluence on oil or water movement. Tidal Channels
Tidal channels comprise a special
192
Tidal Flat
Figure 32-Cross section based on three cores across channel bend shown in Figure 3 1 . Scale in feet (8 It = 2 . 4 m, 240 ft == 73.2 m). Note point bar-like se quence with channel lag g radi ng upward into burrowed intertidal facies. Basal channel lag consists mainly of c e rithid gastropods and locally c on t a in s abun dant fragments of dolomite der ived from erosion of levee as shown in next two figures. N ote that supratidal marsh sedi· ment at base of core through levee (A ' ), as well as entire seq u ence inc l u di ng channel levee, have undergone erosion as channel migrates from right to left. Given stable sea level and sediment sup· ply and time , t he entire intertidal zone could be reworked by tidal c hannels many, many times, t h us concentrating dolomite which forms on levees, as well as converting a relatively i mpermeable i ntertidal flat to a more p ermeable blanket deposit. See Shinn and others ( 1 969) for more detai l.
Figure 33-View of e roded channel levee on Andros tidal flat in the Bahamas. Dolomitic c rust is unde rmin ed and works its way to bo ttom of chan nel du ring lateral migration.
Fig ure 34-A channel levee in the Per sian Gulf (Qatar Peninsula; see Sh inn, 1 973a, for more detail) In this locality th e levee is composed of cerithid gastropod sand and the cementation is by beachrock p rocesses rather than do lomitization . B u rrowing crabs have modified sediment in bottom left on the point ba r , but some c rossbedding may be preserved in the subtidal parts of t he c hannel sequence.
PL EISTOCEN E
Ls
240 ' CHAN N E L 9 E OI M E N T COM B i NF D PON D A N D i N T E fZT I DAL 9 E DJ M E N T
I
LEVEE
lidal Flat
1 93
Figure 35- Three cores of basal chan nel accumulahor"!s from the modern tidal flats on Andros Island, Baharne�s. Cerittlid gastropods and soritid foraminifera are the main sand-sized g rains in cores A. 8 and C. Fragments o1 dolomite are mixed in. and upward grading is appa rent . Note that inclined bedding 1n middle part of core G has e::;ca.peu tJiuturbalion . Dark ooj ecls are mangrove roots.
and dynamic subenvironment of the subtidal zone. Tidal channels range in depth from 0 to 15 m or more along the Persian Gulf Trucial Coast and 0 to 3 m on Andros Island in the Bahamas. All channels, whatever their maximum depth , will become shallower in a landward direction to the point where they disappear. Coring and subsurface observations at Andros Island have shown that tidal channels migrate laterally in a
manner similar to fluvial systems (Shinn et al, 1 969). This similarity led to the concept that highly channeled tidal flats are analogous to river deltas turned wrong side out, that is, the sea, rather than the land, is the sediment source and channels with their distributaries branching in a landward direction provide the pathway for sediment delivery. When tidal channels meander and migrate laterally, a "point bar" ac-
cumulation , similar to but different from that of fluvial channels , is laid down. With continued migration , a blanket of tidal channel sediment may be spread over a large area. Given the conditions of time and lack of seaward accretion or landward onlap, tidal channels could completely rework the intertidal zone many times over. The major difference between sedimentary structures produced in
194
1idal Flat
Fi gu r e 36-Evapo ration ar.d a continual supply of sea water from a fluctuating g round water table lead to concentration of b rinel'> near supratidal su rface. Do lom itization and formation of evapori tes result from these special con d itions found only in the supratidal zone just centimeters above mean tide level. Such areas are also recharged at feast twice a month during spring Ude flood ing . U nder humid conditions at An d ros I s land, evaporites, such as gypsu m , a re periodica lly d i ssolvA d away b y rain and m o r n i ng dew.
Figure 37-Detail of gyps u m precipitated on the Trucial Coast tidal flats i n the Persian Gulf. Such gypsum forms a m ush at least 30 em t h ick above a distinctive algal-laminated zo ne in the
upper inte rtidal zone. See F i gure 3 for relationships. This mush grades laterally landwa rd into layered, nodu lar, often cont o rted anhydrite. S ee next fig u re.
for the action of marine burrowing numerically abundant foraminiferal organisms. The observed sequence at assemblages with limited species Andros Island depicted in Figures 3 1 diversity (Fig. 35) . Most primary bed carbonate tidal channels and those in and 32 starts at the bottom with ding has been destroyed by burrowing siliciclastic fluvial deposits is the clasts of sun-dried supratidal sedi organisms. It should be pointed out, presence of burrowing. The classic ment, fragments and slabs of however , that although bioturbated fluvial "point bar" accumulation supratidal dolomitic crust, and eroded bedding may be difficult to see \n an starts with a basal cobble or pebble bits and pieces of the underlying 80-cm-diameter core, it might be layer (including sun-dried clay in Pleistocene bedrock. Dolomitic and more obvious in larger outcrops. traclasts), which grades upward into non-dolomitic crusts which form on Migrating channel deposits, as shown coarse-grained, festooned crossbed channel levees , as shown in Figures in Figure 32, are eventually capped by ding, followed by finer-grained planar 33 and 34, provide a local source of burrowed intertidal sediments, and bedding containing "clay drapes" lithoclasts for these channel lags. with continued lateral migration, non (thin layers of clay deposited during These clasts occur in a matrix of burrowed laminated supratidal quiet conditions between floods). cerithid gastropod sand which grades sediments will cap the sequence . upward into finer-grained carbonate Similar accumulations might form in In the modern example at Andros tidal carbonate channels were it not sand and packstone containing Island, the underlying bedrock
lidal Flat
195
Figure 38-A pit dug in the Trucial Coast sabkha showing white anhydrite resting on dark algal sediment at base of pit. Salt has precipitated on surface of algae after pit was dug. See next figure for detail of anhydrite. Giving scale to the photo is Godfrey Butler, to whom the author is indebted for acting as axpart guide to the area.
Figure 39-Detail of sabkha trench shown in previous figure showing white "chicken wire" anhydrite at base and interbedding of quartz-rich dolomite and contorted anhydrite layers. Anhydrite also occurs as blebs and lenses.
prevents downward cutting beyond 3 m; along the Trucial Coast, however, some channels that have cut through submarine cemented crusts extend down to a depth of 18 m or more. The age and composition of the underlying pre-Holocene bedrock could not be determined, but in some channels the bottom was strewn with huge slabs of Holocene submarine crusts up to 30 em thick and more
than 1 m across. The important aspect of carbonate tidal channel sediments, whether bur rowed or not, is that their basal parts are porous and permeable and sur rounded by relatively less porous and less permeable tidal flat sediments. They extend more or less perpen dicular to the supratidal and intertidal belt and, thus, may serve as pathways for the migration of interstitial fluids.
DIAGENESIS Dolomite
Both transported and indigenous diagenetic minerals can occur on car bonate tidal flats. The most economically important mineral is dolomite because of its common association with enhanced porosity and permeability. Dolomite not only forms diagenetically on supratidal
196
Tidal Flat
Figure 40-Contorted diapiric anhydrite truncated by sabkha deflation surface. In this arid climate winds deflate sediment down to permanent capillary zone, where permanently wet sediment resists removal. No t e that sediment has been added subsequent to erosion. Position of the capillary zone varies depending upon season and climatic variations. Note numerous nodules of anhydrite. Gypsum also occurs as large scattered rosettes.
ratio to many times that in normal sea water. Changes in pH associated with respiration and photosynthesis of blue-green algae, as well as oxida tion and the pumping action of fluc tuating ground water, may also be contributing factors in supratidal dolomitization. Regardless of the ex act chemistry, this kind of dolomite is readily identified in modern and an flats but, as discussed earlier, detrital cient rocks because of its association dolomite can find its way to these en with distinctive sedimentary structures and its small crystal size. Modern. vironments through eolian processes . tidal flat dolomite, often called ''pro Much has been written concerning todolomite" because of poor ordering dolomitization on modern tidal flats and high Ca-to-Mg ratio , is universal (Wells, 1962; Illing et al, 1 965; Def ly composed of small crystals ranging feyes et al , 1 965; Shinn et al , 1 965; in size from 2 to 4 ,.m . In similar an Shinn et al, 1 969; Hardie, 1 977; Kinsman, 1964; McKenzie et al, 1 980, cient tidal flats, dolomite crystals are to name a few). Although the also small, but generally in the 5 to 1 0 ,.m size range, and the crystal lat chemistry of dolomitization remains controversial, many of the controlling tice is well ordered . Larger dolomite crystals do occur, however, especially parameters are known, especially for supratidal dolomites which are by far in associated marine or channel the easiest to recognize in the geologic deposits, where initial permeabilities record . Dolomite tends to form at the were high. Whether the chemistry of ancient tidal flats was different , or surface on supratidal flats a few cen whether poorly ordered high-calcium timeters above normal high tide. In dolomite becomes ordered with age is this setting, underlying sea water is not known. brought to the surface by capillary Dolomitization probably also oc action , or is supplied by surface curs by seepage of Mg-rich surficial flooding during storms, and brines into underlying supratidal, in evaporates to form a highly concen tertidal and subtidal sediments, as trated brine (Fig. 36) . Through precipitation of gypsum (CaS04) and proposed by Adams and Rhodes (1960) . Deffeyes and others ( 1 965) aragonite, calcium is selectively removed , thus elevating the Mg-to-Ca believe that much of the dolomite in
sediments underlying the salt pans on the island of Bonaire (Dutch Antilles) was formed by this mechanism. Steinen and Halley ( 1 979) have reported protodolomite beneath a Florida Bay mud island that they believe may have formed by refluxa tion. An example of subsurface primary (non-replacement) dolomitization in brines beneath a porous quartz sand sabkha in the Persian Gulf was discussed by Shinn ( l 973b) and DeGroot ( 1 973). Detrital dolomite, quartz silt and various clay minerals are present in most Persian Gulf sediments. The detrital dolomite contaminates not only areas of supratidal dolomitiza tion but also practically all marine sediments of similar grain size. Pilkey ( 1966) discovered l OOJo detrital dolomite in fine-grained subtidal sediments from the central axis of the Persian Gulf. Such occurrences are not surprising. During offshore work in the Persian Gulf, the author's research vessel was often covered in a matter of hours with dolomite, clay, and silt dust resembling yellow-brown paint. Some of what had been reported as modern dolomite from Persian Gulf flats was most likely of detrital origin. It seems likely, therefore, that detrital dolomite should be abundant in many ancient tidal flat settings, especially those deposited during particularly arid ,
lidal Flat
Figure 4·: -Ar;a.1 cMa.j and f1J:ed tidC?.I �r.an f'l f) li� f ab<.<:1rJvr1rnt:n� ;;,nc SCd1rnO:"\ ���Y �aqL:en�- �) . CarT-e: �rads in fore grour.::l. rht'ise Suti�le cl'a.""lnlill� can l':l-3 rac�c: s�a.ward tc presenfoi' act:ve
c<1a.1ne1s. Port:(:Jl o? J 15-m-lo'"t£1 by 2�1l·�P.ep t!"ench (B) d..;g across an aban· dorre.l �:.�;Wrr)..;; :ii"!:lel seqt.:ence '-'>'ith mo�e porcus
�idat
197
�t,d pt:ur�>ibl� �edirm:::l:; :al ;he lop. Netase of net�c:-: belov• rr;arYs hand. fVa,-: i$ st�o,'1(1i:"ll: on wa;f.\r cov0-rec cnrrrentec �rust. which l<ept trench dry unt:l cru.st .... �s per;Gtra;ed w·th a rock harnmer.
198
lidal Flat
Figure 42-ldealized tidal flat model combining onlap and offlap features of Andros Island and Persian Gulf tidal flats to show stratigraphic traps and related porous accumulations. Trappi ng condi tions occur where inte rtidal and channel facies p in c h out beneath and within relatively impermeable supratidal facies. Supratidal facies in most ancient ex am ples are sealed with anhydr i te and other evapo r it i c minerals. Additional
reservoi r development may occur
in tidal
deltas, channel and barrier island sands that l ie seaward of su prat idal and con
tinental facies. In most ancient examples supratidal facies are composed of relatively i mpermeab le micro-dolomite. whereas subtidal facies tend to be com posed of more permeable, coarser g rai n ed dolomite.
+ ... +
COASTAL PLAIN (r:on-porou::s
�·.,.�prit.tidal anhydritic d-olcmitt�o) !
00 0 0 0
SHALLOW MARINE (porous
--
--
dolo�Y-;t.,i
!
I
------ --r- ? DEEP MARINE
(dark l imestone 1\nC .chen.y
DELAWARE BASIN
1
dvt<.�fllil�)
I
0 u
.>< w ::?. "' 5 2
Figure 43A-An exa mple o� a tidal flat system developed o;uallel to the n o rth ern margi n of the Delaware Basin (Middle Permian San Andres t i m e). Section X-X' is shown in F ig u re 438. Illustration courtesy of Fred Meissner ( Bir d Explora tion Com pany).
0
(f) .J. i X '
I�
I
20 MIL�S
k::;ilr:si -
0
20 KH.OME ,.ER<:;
Tidal
SOUTH X
------
L
� M I E�
Flat
199
NORTH --·
--------·---·-·-· ---- ·
x·
MILNESAND FIELD SUN
SCHI\'.IuT
SUNRAY. t-tolJ
SUN. COSBY
LL
LL
rt:E I VcR II CAl SCALE
� ��
Swprc:tlltl�d
CJ
Open marone
� lntertJdal and s.ubt1dal � li::lke �nd chc:-u-:n.cl
JO
::
�� �T
HORIZONTAL SCAlE
U
TcXA�. CHAMPeAU LL
Potostty Interpreted from latera.lo�
Coofin'!fd tu open rr:�r.ne and tldal char.n�/:';;J
ARBITfi:At-(Y
Figure 438-North-south cross section
showing three reservoirs sealed by im
permeable anhydritic supratidal facies to the north. Several of the fields along the irend shown in Figure 43 owe their existence to a combination of hydrodynamic flow from the west and the 1orthward pinchout of poros:ty ;n the supratidal anhydritic dolomites. Tre largest of these fie!ds, Slaughter Levelland, contains 3.5 billion barrels of oil in place (Fred Meissner, personal commun., 1981).
periods, such as the Permian . Gypsum and Anhydrite
Some evaporitic minerals, such as halite, anhydrite , and gypsum, are present on all modern arid tidal flats. Many ancient tidal flats contain abundant anhydrite and salts, especially those of Permian age. Gyp sum may form temporarily on humid tidal flats like those at Andros Island in the Bahamas. It is often present during the dry season but disappears during the wet season and therefore is not preserved for the geologic record. Ancient tidal flats that do not contain evaporitic minerals could have form ed by this mechanism during humid periods of the earth's history .
On the Persian Gulf tidal flats, where annual rainfall is less than 2 em, a gypsum mush, composed of crystals resembling ice cream salt, forms an extensive layer up to 30 em thick. The gypsum mush forms in the transitional zone between intertidal and supratidal conditions (Figs. 3, 37; Kinsman, 1 964; Butler, 1 965; Kendall and Skipwith, 1 969) . The gypsum layer grades laterally to contorted, fine-grained anhydrite (Figs. 38, 39, 40) farther landward, where it is overlain by supratidal dolomite, anhydrite, and quartz sand . The lower part of the buried anhydrite zone contains the familiar "chicken wire" texture so prevalent in many ancient gypsum and anhydrite deposits. The chicken-wire texture grades upward into distorted layers of fine-grained anhydrite (Figs. 38, 39, 40). How much of the anhydrite is primary and how much has formed by replacement of gypsum is not well known . Farther landward, where the sabkha is subject to flooding by rain water outwash or influx of continen tal groundwater, anhydrite has locally been hydrated back to gypsum (Butler , 1 965 ; Kendall and Skipwith, 1 969).
Gypsum also occurs as large isolated rosettes several centimeters across . Such crystals are randomly scattered throughout the intertidal and algal mat zone underlying the anhydrite. Gypsum rosettes increase in abundance and size toward land and appear to have a different origin than the gypsum mush or anhydrite. Celestite, a temporary evaporite mineral, is sometimes associated with anhydrite. Since this mineral is not preserved, however, it is considered unimportant for the purposes of this article . Cementation
Cementation of sediment occurs in four main areas on tidal flats: ( 1 ) as surficial crusts, often dolomitic, on the lower parts of supratidal flats and natural levees; (2) along the edges of tidal channels in areas where channel banks are composed primarily of car bonate sand (Fig. 34); (3) as beachrock in the intertidal part of beach ridges and spits; and (4) on nearly horizontal, sandy flats in the upper part of the subtidal and lower part of the intertidal zones . Of the four kinds of cementation, the last three can be considered forms
200
Tidal Flat
facies frorn Per miar, Slaughter-Leve!land-Millensand trend. (A) Supratidal 11odular anhydrite facies. (B) Supratidal anhydrite and algal stromatolite facies. (C) Burrowed, high energy int€rtidal or channel f<'lcies (D) Figure 44-Selected
Leached skeletal sha!low-marlne
dolomite wackestones. (E) Open-marine, porous brachiopod and echinoid spine lime wackestone.
dry even though the bottom was below sea level. When the crust was broken, however , water gushed up to fill the trench to a depth of approx imately 30 em above the crust. No leakage through the unbroken crust was observed , although it was expos ed in the trench for a distance of 15 m. Such crusts, therefore, not only limit channel scour but undoubtedly have a controlling effect on such diagenetic phenomena as refluxation
or capillary movement of brines. Submarine crusts with low permeabilities are also forming far ther offshore in the Persian Gulf (Shinn, 1969) . Given time, many por tions of the various Persian Gulf tidal flats will accrete seaward over these submarine cemented crusts . Thus, the tidal flat system could incorporate and be affected by yet another permeability barrier. Similar crusts, recognizable mainly by fibrous calcite or dolomite cements, are undoubtedly a component of many ancient tidal flats. They can easily be misinter preted as exposure unconformities because of their bored and eroded surfaces .
Tidal Flat
STRUCTURE MAP DATUM:TOP PHOSPHORIA C.i.•IOOO'
R91W
R90W
I 0\ 0 0 0
R90W
201
T 48 N
T 48 N
T 47 N
COTTONWOOD COTTONWOOD
A
HYPOTHETICAL TIDAL CHANNEL PATTERN AT A GEOLOGIC MOMENT OF ERVAY TIME
CR. FIELD
9.6 km
0 .ILES F=====-=��'F='===i
Figure 45A-Structure map on the top Ph os pho ria Formation in the Cot tonwood Creek Field in the Bighorn Basin of Wyoming. Note dry holes updip to the northeast and producing wells downdip to the southwest. The field had produced 37 mil lion barrels of oil as of 1971 (from Rogers, 1 97 1 ).
B
OF===6==9F�M�IL�Es==9F====�9. km
of the
Hypersaline Caliche and Pisolites
Laminated crusts, similar to fresh water vadose caliche crusts of the American west and those described from Florida by Multer and Hoff meister ( 1 968) and Robbin and Stipp ( 1 979), occur in several supratidal areas of the Persian Gulf. Pisolites are associated with the crusts (Purser and Loreau, 1 973) or as individual pisoids floating in uncemented supratidal sediment (Shinn, 1973b). Scholle and Kinsman ( 1 974) demonstrated that these features are, in essence, vadose caliche, but instead of owing their formation to fresh water, these formed by precipitation from hypersaline waters. Scholle and Kinsman ( 1974) also showed evidence that these features are modern analogues to the well known pisolites and pisolitic crusts associated with lagoonal and tidal flat facies con nected with the Permian Capitan limestone, a reef complex of west Texas and New Mexico . Similar
CR. FIELD
pisolitic facies associated with tidal flats are common in the subsurface platform areas of the Permian Basin of west Texas (Longacre, 1 980) . In modern environments of the Persian Gulf, laminated crusts and pisolites occur in supratidal en vironments on Pleistocene bedrock (Scholle and Kinsman, 1 974) and high intertidal-to-supratidal areas on beachrock (Purser and Loreau, 1 973) or in unconsolidated siliciclastic sedi ment in the low supratidal zone (Shinn, 1 973b). In all examples, the site of formation is in the seaward portion of the sabkhas exposed to sea spray and spring or storm tide flooding. If their origin in ancient tidal flats is similar, then one can assume that their presence may be a useful indicator for predicting facies relationships. Ancient pisolites and pisolitic crusts, usually associated with teepee structures in the Permian rocks, are suspected to have formed within 2 km of either a subtidal lagoon or the sea. Evaporites and Silica Cement
Evaporites, either as rosettes of gypsum or banded anhydrite (Figs. 38, 39), can be considered a cement . These evaporites may cause some cementation of sediment by infilling
Figure 458-Facies interp retation of the Park City Formation in the Cottonwood Creek Field showing porous channel deposits extending from downdip permeable intertidal and marine rocks into updip impermeable evaporitic supratidal facies. Awareness of the possibility of channels in tidal flat reser voirs can aid hydroca rbon exploitation.
of bird's-eye vugs or, in some cases, produce a hard rock by infilling in traparticulate void space. Whether or not these cements produce a hard rock, they are nevertheless important , because they can effectively block porosity and permeability, and they can flow rather than fracture during compaction or tectonic deformation. Supratidal sabkhas containing evaporites can therefore form effec tive seals. If leached by fresh water during later diagenesis, excellent porosity and permeability may be produced. Opaline silica, or chert , has been repeatedly observed in ancient supratidal sediments, although true silica cement has not yet been reported from modern tidal flats . Silica cements in tidal flats are prob ably not important, as they are seldom extensive enough to form ef fective permeability barriers .
202
lidal Flat
;.,. � 0 - ... 0 0 .r:;0 u
Figure .t6-Core from a portion of the Permian San And•es colomite ln west Texas showing facies detail that can be extracted rr •Jm tidal fi<:H seq:Jences. At least 18 supratidal zones (indicated by red cofo; ln the environmental interpreta· tion column} were logged in this core. In· tertidal and/or marine rocks are show-1 in blue , ancJ root zones (either intertidal or high supratidal} are indicated by green. Minor t,;nconforrnities are the
RIDLEY�WHEELER, EDWARDS
no.1
rule. Rec:ause of exposure associated
with tidal flat accumulation, s i gnificant portion3 of tia'al flat sequences arc prob ab!y eroded away before deposition of
the next ,mit, thus perfect cycles, s ch as shown In R�ure 5 0, are prol:>aoly rare in the geologic record. Notice pers'stent col o r changes from oxidized brown supratidal dolomite to gray subtldal
dolomites.
DISCUSSION AND ECONOMIC IMPLICATIONS
Porosity and permeability in tidal flat systems vary greatly from facies to facies. The various studies of modem and ancient examples com bined suggest that porosity and permeability are best developed in subtidal-to-intertidal facies with greatest development in tidal chan nels. Supratidal sediments appear to offer the least potential for porosity and permeability development. The relative lack of porosity and permeability in this environment can be attributed to susceptibility to (1) early cementation, (2) formation of micro-crystalline dolomite {often associated with cementation by aragonite and Mg-calcite), (3) forma tion of evaporites, and (4) the development of algal mats.
root zones s�Jpratidal - Intertidal andior marine·r
0a-
Structural Traps
Knowledge of permeability varia tions and recognition of the facies that contain these variations can be useful to the subsurface explora tionist, whether he or she is searching for structural or stratigraphic traps. The best documented ancient example involving structure is in the Stony Mountain Formation (Ordovician) and Interlake Formation (Ordovician and Silurian) in the western Williston
Basin, described by Roehl (1967). The Cabin Creek oil field is an anticlinal
structure that uplifted rocks represen ting all common tidal flat facies,
thereby making identification of those zones most economically exploitable an important criterion in the search for oil. Predictability of facies was
£:1- dolomite
•
-
anhydrite
good in the Cabin Creek Field, but predictability of porosity associated with the facies was poor, probably because early subaerial exposure left a diagenetic porosity overprint on all facies. Roehl ( 1 967) did note that, although all the facies are dolomitiz ed, the larger crystal size and hence
lidal Flat
203
Figure 47A-Photograph of t h e Cam b r ian Carrara Formation in N evada demonstrating the persistence of color in supratidal and subtidal carbonates. The light-colored oxidized unit at the top of the cliff overlies gray reduced marine rocks. Such color changes are thought to be usef u l in inte rpreting tidal flat rocks of all ages. These color changes can be useful indicators to the subsur face geolog ist working only with cut· lings. Photo courtesy of R. B. Halley (U.S. Geological Survey). For detai ls, see Halley (1975).
the greatest porosity, up to 2007o, and permeability of approximately 2.5 md were associated with interconnecting burrows in subtidal and intertidal facies. Sediment contained in modern bur rows is generally coarser-grained than the surrounding sediment (Shinn, 1968c). Filled burrows can therefore channel fluids and become dolomitiz ed more quickly than the matrix. Such dolomite is likely to be of larger grain size than that in the surroun ding matrix. Highly burrowed zones, as noted earlier, are restricted to sub tidal ("infratidal" of Roehl, 1967) and upper intertidal facies. Coarse grained channel deposits were thought by Roehl (1967) to occur within the intertidal zone. Even the supratidal zone, especially where it contained abundant intraclasts, also contained oil and sufficient porosity for produc tion, but this was interpreted to be the result of early subaerial leaching. It should be remembered that tidal flats flanking a continually subsiding basin would be less likely to ex perience exposure and leaching. Based on the Holocene models (Andros Island and the Persian Gulf) and excluding the effects of early ex posure and leaching, the best chances of porosity development lie in sub tidal marine, channel and intertidal deposits. Early cementation and
evaporite development leave supratidal sediments with the least potential for reservoir development. If tidal flat rocks have been struc turally uplifted, one should expect maximum reservoir development where initially more porous zones have closure. The potential for multi ple reservoirs is enhanced even more if several impermeable supratidal zones separate and seal structurally uplifted porous and permeable sub tidal and intertidal facies. Such knowledge could be additionally im portant in tidal flat oil fields, where secondary recovery is necessary. When structurally deformed, inter tidal and submarine-cemented crusts, such as those discussed earlier , prob ably would not provide effective seals because of the ease with which these brittle rocks fracture. Evaporitic supratidal zones, however, would be more plastic and less likely to fracture and leak. The Stratigraphic Trap Model
Tidal flats are basically a special kind of shoreline deposit and, as such, are subject to repeated trans gressions and regressions (onlap and offlap) and subsidence through time. On the flanks of subsiding epirogenic seas and basins, these special shoreline deposits have the potential to produce predictable belts of reser-
voirs and seals. The simplified con ceptual model shown in Figure 42, based on studies of modern and an cient examples, shows the relation ships between porous and non-porous facies, as well as trends in relation to the basin or adjacent land mass. Meissner (1974) described the hydrologic and stratigraphic trap con ditions controlling the Milnesand and Slaughter-Levelland fields in the Per mian San Andres dolomite of the northern Delaware Basin in New Mexico and Texas. According to Meissner (personal commun . , 1981), these originally discrete fields are now interconnected and controlled by an impermeable east-west supratidal and continental facies to the north. Oil migrated from the deeper Delaware Basin in the south and became trap ped in a belt of more porous and permeable subtidal-to-intertidal sediments that pinch out to the north within and beneath the impermeable supratidal (sabkha-type) facies. The relationship of these facies is shown in Figure 43A and B. Core slices showing the major facies are shown in Figure 44A through E. Rogers (1971) described tidal flat facies and its control on reservoir and trap strata in the Permian Phosphoria Formation Cottonwood Creek Field in the Bighorn Basin of Wyoming. Rogers' geologic interpretation is
204
Tidal Flat
Figure 478-Cioseup of the oxidized algal-laminated and mudcracked supratidal zone shown in previous figure (from Halley, 1975)
similar to that of Meissner (1974). At supratidal and intertidal rocks from Cottonwood Creek, production (over gray subtidal rocks. Figure 47, a 37 million barrels of sour crude have photograph of the Carrara Formation been produced from nearly 100 wells (Cambrian) in Nevada described by since the field's discovery in 1953) is Halley (1975), shows that even in from grain-supported dolomitized in these very old rocks, the supratidal tertidal facies, including the deposits zone can still be distinguished by its of tidal channels. The seal is an oxidized color. Longacre (1980) described the com evaporitic supratidal zone to the bined structural and stratigraphic northeast (Fig. 45A). Porous fingers penetrate into the non-porous aspects of the Permian Grayburg and supratidal zone, as interpreted in Queen Formations of the North McElroy Field on the eastern side of Figure 45B (Rogers, 1971). Rogers pointed out that tidal flat facies could the Central Basin Platform of the be identified in cuttings but that cores Permian Basin in west Texas. At McElroy Field, platform margin were necessary to determine the ver tical facies sequence. The author has faulting provided the pathway for oil logged cuttings and cores from migration from basinal source rocks several wells penetrating Permian San into the numerous onlap and offlap Andres tidal flat rocks in west Texas tidal flat sequences on the platform. which support Rogers' conclusion. The seal is an anhydritic supratidal When the geologist is working with facies, whereas the reservoir rocks are these kinds of rocks, recognition of "shallow shelf" and "exposed flats" facies relationships from a few cores, (presumably synonymous with sub tidal and intertidal flats) that pinch aided by electrical logs, should allow a rewarding use of cuttings if the out to the west beneath supratidal facies (Longacre, 1980). Selected ex wells are closely spaced. A large amples from the North McElroy Field amount of facies detail can be ex are shown in Figure 48. tracted from cores, as shown in a 22 McElroy-type reservoirs are prob m (70ft) core in Figure 46. More than 15 supratidal zones were ably repeated in many locations along the eastern and western sides of the recognized in the core shown in Central Basin Platform. Flannigan Figure 46. Subsequent study of cut Field farther to the north is a good tings from similar rocks showed that the characteristic oxidized brown col example of a reservoir developed in tidal flat facies (Jerry Lucia, personal or was especially useful in separating
commun., 1981), as are numerous fields flanking the northern edge of the Delaware Basin (Allan Thomp son, personal commun . , 1981). Many of these fields are near deple tion and have been undergoing water flood and other secondary recovery programs for several years. Although seldom mentioned in geologic literature, exploration geologists often discuss the problems encountered by production engineers during water flooding. Basically, the problems have resulted from lack of recogni tion by petroleum engineers of the numerous horizontal permeability barriers. Understanding of tidal flat sedimentation and recognition of the facies variations, therefore, would be very useful to petroleum engineers. Outcrop Examples
Similar transitions from rocks dominated by supratidal facies to those dominated by subtidal and in tertidal facies have been reported from outcrop studies of rocks of Or dovician age in Kentucky (Cressman and Noger, 1976), (Fig. 49). Although environmental interpretations are slightly different, Howe (1968) pro vided a similar interpretation of lateral pinchout of tidal flat facies in Cambrian strata of the St. Francois Mountain area of Missouri (Fig . 50). The geological literature abounds
Tidal Flat
f-Igure 4fl- E xamp,es ol tidai llat fac1es fri)!Yl �he Permia:'l Graybur� Me Queen Fo1 ma!luns of I he NO·"th Me -1roy Fidd on tha eas: side of t11e Centlal Baaln Pl:.t!for:n of !he Pcrm!ar Bas•n. wast Texas. (A·D) Examples of •ar:ous s-..:Yatlclal fa.:;�s with d:agnostic :eatures SlJGh as algal rnat:5. gypsum �rrslal�. nodular anhy'drlt:e, and biro's-eyas. (E) Aeprasal1ta(ive subtidal lacl€s Cor�g courlf:sy S>Js;Jn vflgHc:re;;
{Getly Exploratio:'!).
205
206
lidal Flat
North A
B
c
D
E
South F
Figure 49-ldealized north-south stratigraphic cross section through the Ordovician Tyrone Limestone in Ken
tucky. Note nurnerous stra!igrapt1ic pinchouts, where intertidal and subtidal rocks pinch out against supratidal dolomite. Multiple reservoirs could be
developed if the burrowed subtidal or in· tertidal rocks should be porous and permeable. Supratidal rocks would form seals (from Cressman and Noger, 1976).
with other examples of tidal flat rocks not cited here, one of the more notable being the Lofer tidal flat cycles of the northern Italian Alps, described by Fischer (1964). Examples for this paper were chosen mainly on the basis of economic importance and regional stratigraphic extent. Sedimentary and Diagenetic Features for Identifying Ancient Tidal Flats No single sedimentary structure
w ill
11\
so
•5-
It
40
!0
EXPLANATION
30
Supratidal rocks
c=J
20 -10
0
serve as the key to unlock the secrets
.... 5
ltm
10
15
10
� '0
0
0 0
Intertidal rocks Subtidal rocks Bentonite
or even the presence of ancient tidal
flats. Recognition of the sequence of sedimentary structures and facies commonly developed in tidal flats is probably more important than any single sedimentary feature . Figures 51 and 52 are attempts to combine what is known of both modern and ancient tidal flats into a usable sequence of sedimentary structures. Figure 5 1 shows the sequence and subtidal variations of the Persian Gulf-type of tidal flat, and Figure 52 shows impor tant attributes of all known tidal flat cycles combined. It should be made clear that numerous and important variations can occur. M a n y of these variations are indicated in the onlap offlap model in Figure 42 and in the sedimentary sequences shown in Figures 51 arid 52. What the geologist encounters in any geologic basin is likely to differ, depending upon struc tural style, climate, age and latitude .
No attempt has been made to show combinations of structural and diagenetic alterations that can oc cur. Such an effort is probably not possible and would be beyond the scope of this chapter. As in most geological problems, each area will have its own peculiarities and varia tions that can be understood only through thorough geological study . The models provided here are intend ed to serve as bases for geological departure. Modifications for any par ticular area will be necessary for bet ter understanding, discovery, and ex ploitation of this economically impor tant sedimentary system. all p ossible
ACKNOWLEDGMENTS The author gratefully acknowledges the
constructive suggesti ons and discussions
B. Halley and Peter A. Scholle of the U.S. Geological Survey. Special appreciation is extended to Bar bara Lidz for editing, preparation of il lustrations and continual review during all stages of manuscript preparati on. The field work was done and many of the
with Robert
ideas expressed here were developed when the author was in the employ of Shell
Development Company, USA, and the Royal Dutch Shell Laboratory in the Netherlands. The following people provided either sam ples or slides that contributed greatly to the quality of the paper: Jack Moore of Shell Development Company (two figures); Earl Cressman of the U.S. Geological Su rv ey (s everal col or
Tidal Flat
Figure 50-The potential for several pay zones is apparent in this stratigraphic in terpretation of Cambrian rocks in Missouri described by Howe (1968). What Howe called planar stromatolite units is interpreted by this author to represent offlap supratidal marsh or sabkha equivalents and thus might serve as seals. Reservoirs would likely be restricted to the burrowed mud unit in terpreted to be undifferentiated intertidal and s u bt idal rocks. provided the entire section dips toward the basin.
LAMOTTE SolNOSTONE
photographs); RobertN. Ginsburg of the University of Miami Fisher Island Station (slides); Clyde Moore of Louisiana State University (samples of Cretaceous tidal flat rocks); Dennie Cody for preparation of color prints. Godfrey Butler of Exxon Company, USA, introduced the author to the Trucial Coast evaporites and served as guide to the area. Fred Meissner of Bird Exploration Company provided material and information concerning oil produc tion from tidal flat rocks. Edward Even son of Lehigh University provided the soilstone crust in Figure 16B. Susan Longacre of Getty Exploration provided
samples from North McElroy Field. Appreciation is also extended to Daniel M. Robbin and J. Harold Hudson of the U.S. Geological Survey Fisher Island Sta tion for encouragement and discussions during manuscript preparation .
SELECTED REFERENCES Adams, J. E., and M. L. Rhodes, 1960, Dolomitization by seepage refluxion: AAPG Bull., v. 44, p. 1 912-1 920. Ball, M. M., E. A. Shinn, and K. W. Stockman, 1 963, Geologic effects of Hurricane Donna: Abs., AAPG Bull., v. 47, p. 349. Black, M . , 1933, The algal sediments of Andros Island, Bahamas: Phil. Trans. Royal Soc. London, Series B, v. 222, p. 165-192. Butler, G. P., 1965, Early diagenesis in the recent sediments of the Trucial Coast of the Persian Gulf: London Univ ., Ph.D. dissert., 251 p. Cressman, E . R . , and M. C . Noger, 1 976, Tidal-flat carbonate environments in the High Bridge Group (Middle Ordovi-
207
cian) of central Kentucky: Lexington, Univ. Kentucky, Kentucky Geol. Survey, Series X, Rept. of Invest. 18, p. 1-15. Davies, G. R., 1970, Algal-laminated sediments, western Australia, in B. W. Logan et al, eds., Carbonate sediments and environments, Shark Bay, Western Australia: AAPG Mem. 13, p. 169-205. Deffeyes, K. S., F. J . Lucia, and P. K. Weyl, 1 965, Dolomitization of Recent and Plio-Pleistocene sediments by marine evaporite waters on Bonaire, Netherlands Antilles, in Dolomitization and limestone diagenesis: SEPM Spec. Pub. 1 3 , p. 71-88. DeGroot, K., 1 973, Geochemistry of ti dal flat brines at Umm Said, S.E. Qatar, Persian Gulf, in B. H. Purser, ed., The Persian Gulf-Holocene car bonate sedimentation and diagenesis in a shallow epicontinental sea: Heidelberg, Berlin, Springer-Verlag Pub., p. 377-394. Evans, G., 1965, Intertidal flat sediments and their environments of deposition in the Wash: Quart. Jour., Geol. Soc. London, v. 121, p. 209-245. __ 1 966, The recent sedimentary facies of the Persian Gulf region: Phil. Trans. Royal Soc. London, Series A, v. 259, p. 291-298 . ___ et al, 1969, Stratigraphy and geo logic history of the sabkha, Abu Dhabi, Persian Gulf: Sedimentology, v. 12, p. 1 45- 1 59. Fischer, A. G., 1964, The Lofer cyclo thems of the Alpine Triassic, in D. F. Merriam, ed., Symposium on cyclic sedimentation: State Geol. Survey Kan sas Bull. 169, v. 1, p. 1 07-149. Garrett, P., 1 970, Phanerozoic stromato lites - non-competitive ecologic restric-
tion by grazing and burrowing animals: Science, v. 169, p. 171- 1 73. Ginsburg, R .N., 1 975, Tidal deposits, a casebook of Recent examples and fossil counterparts: New York, Spring-Verlag Pub., 428 p. Grover, G. Jr., and J. F. Read, 1978, Fenestral and associated vadose diagenetic fabrics of tidal flat car bonates, Middle OrdovicianNew Market Limestone, southwestern Virginia: Jour. Sed. Petrology, v. 48, no. 2, p. 453-473. Hagan, G. M., and B . W. Logan, 1 975, Prograding tidal-flat sequences - Hut chinson Embayment, Shark Bay, Western Australia, in R. N. Ginsburg, ed., Tidal deposits, a casebook of Re cent examples and fossil counterparts: New York, Springer-Verlag Pub., p. 21 5-232. Halley, R. B., 1975, Peritidal lithologies of Cambrian carbonate islands, Carrara Formation, southern Great Basin, in R. N. Ginsburg, ed., Tidal deposits, a casebook of recent examples and fossil counterparts:New York, Springer Verlag Pub., p. 279-288. Hardie, L . A., 1977, Sedimentation on the modern carbonate tidal flats of northwest Andros Island, Bahamas: Baltimore, The Johns Hopkins Univ. Press, The Johns Hopkins Univ. Stud. in Geology, no. 22, 202 p. Hoffman, P. F., 1967, Algal stromato lites-use in stratigraphic correlation and paleocurrent determination: Science, v . 157, p. 1 043-1045. Hofman, H. J., 1 969, Stromatolites from the Proterozoic Ahimikie and Sibley groups, Ontario: Geol. Survey of Canada Paper 68-69, 55 p. Howe, W. B., 1968, Planar stromatolite
208
lidal Flat
D i a g n ostic Sedi mentary Structu res Cross b e d d e d carbonate or q uartz eoli a n sa n d wlt n
Unco nfo r m i t y.
<;-J A n hydrite
mite w ith w i n d b lown quartz .
Figu re 51 -I deal offlap sequence b ased on
the Persian Gulf
examples described
in the l iteratu re and observed by the
au t ho r . Such a c om pl ete sequence is u n l i ke l y i n the geolog i cal re c o rd due to
A n h yd r ite or g y p s u m
(chicken wire text u re).
the poten tial for subaerial erosion during deposition. In addition , seve ra l variations a r e possible. !- i rs!, n ot i ce that eolian sands above the s up rat i d a l zone are like ly and where they occ u r ,
these
diapirs and
contorted layers i n dolo
J
Gra d a t iona l co n tact.
H i g h l y orga n i c algal
laminat ions with b i r dseye
sands
probably blanket and prese rve sabkha facies . Such sands commonly m i g rate over the suprat idal zone duri n g and soon
voids and m u dcra c k s .
L a minations l e s s a l g a l
d ow n wa rd r rading into
after d eoos it ion in arid climates. This movement ·and protection by eol ia n sands is common on mod e r n tida: flats in the Pc rs i n n Gt:lf. Second, n ot i ce that subti dal facies can range (depending uoon local conditions) f rom crossbedded
b u rrowed lime m u d
or
d o l o m ite.
G ra y b u r rowed a n d p e l le ta l l ime mud.
Bored grai nsto n e crust m a y or may not be pre sent .
carbonate sands, oft e n oolitic , to coral reefs . In the geological record corals
@ C ross b e d d e d <: a r bonate
grainstone (tidal bar o r beach ). May be oolitic or sil i ciclastic sand pos s i bly underlain by coral or other open water facies.
mig;,t be replaced by stromatoporids, rud i stids o r other reef b u il de r s , but oolitic fac i e s are c omm o n . Subtidal facies may also be composed or pe!leted and burrowed muds. Cemented crusts wi th bored and eroded u ppe r surfaces may occur both within and at the top of s u bt i da l factes. Ttle r 1 1us t diagnostil.: facies, however. is the desiccated algal stromatolite Lane e:trrd the nodu :ar or
@ Lagoonal facies.
Cray
b u rrow - m o t t l e d l i m e mud (may be dolo mitic).
chicken wi re anhyd rite zone.
and burrowed carbonate mud facies in Cambrian strata of the St. Francois Mountain area: Missouri Dept. Business Admin . , Div. of Geol. Survey and Water Resources, Rept. of Invest . 4 1 , 1 1 3 p. Illing, L . V., 1954, Bahamian calcareous sands: AAPG Bull . , v. 38, no. I , p . 1-95. __ 1959, Deposition and diagenesis of some Upper Palaeozoic carbonate sediments in western Canada: Proc. , Fifth World Petroleum Cong. , Sec . I , Paper 2 . __ A. J . Wells, and J. C. M. Tay lor, 1965, Penecontemporary dolomite in the Persian Gulf, in L. C. Pray and R. C. Murray, eds . , Dolomitization and limestone diagenesis - a symposium: SEPM Spec. Pub. 1 3, p. 89-1 1 1 .
James, N. P . , 1977, Shallowing upward sequences in carbonates: Geoscience Canada, v. 4, no. 3, p. 1 26- 1 36. Kendall, G . St. C . , and Sir P . A. D'E. Skipwith, 1969, Holocene shallow-water carbonate and evaporite sediments of Khor a! Bazam, Abu Dhabi, southwest Persian Gulf: AAPG Bull . , v. 53, p. 84 1-869. Kinsman , D. J. J . , 1 964, The recent car bonate sediments near Halat el Bahrani, Trucial Coast, Persian Gulf, in Deltaic and shallow marine deposits-developments in sedimen tology, v. 1 : Amsterdam, Elsevier Sci. Pub . , p. 1 85- 192. Klein, G . V . , 1 97 1 , A sedimentary model for determining paleotidal range: Geol. Soc . America Bull . , v. 82, p. 2585-2592.
, 1 972, Determination of paleotidal range in clastic sedimentary rocks: Proc . , 24th InternaL Geol . Cong., Sec . 6, p. 397-405 . Logan, B. W. , R. Rezak, and R . N . Ginsburg, 1 964, Classification and en vironmental significance of algal stromatolites: Jour. Geology, v. 72, p . 68-83. Longacre, S. A . , 1980, Dolomite reser voirs from Permian biomicrites, in R . B. Halley and R . G . Loucks, eds . , Carbonate reservoir rocks: Denver, SEPM Notes for core workshop no . l : p. 105- 1 1 7 . Matter, A . , 1 967, Tidal-flat deposits in the Ordovician of western Maryland: Jour. Sed. Petrology, v. 37, p. 60 1 -609. McKenzie, J. A . , K. J. Hsu, and J. F. Schneider, 1 980, Movement of subsur__
Tidal Flat
R EGIO N A L C O M PAR I SON O F S E D I M ENTA R Y F E ATURES
209
R E L AT I VE D IST RI B U T IO N OF STRUCT U R ES , GRAINS A N D M I N E R A L S
..1
< 0
-
....
Cd :I Ill
Figu re 52-An attempt to compile all commonly accepted fac*:lts of facies variations associated with carbonate
tidal f!a l accumulation s . (A) shows the sequenct'! on lldal Hats around the Qat�H Peninsula in the Persian Gu:f. Note lack o1 a welt devc l ooed alga! mat or anhyd rite zor.e due to s l ightly higher rainfal l than on tile Trucial Coast. (B) shows sedimentary structures, such as so i l c : asts . cu rrent-depo sited i ntrac1as�s. m i rwr al®.l)! eads_a!1d. domes. snud. .
face waters under the sabkha, Abu Dhabi, United Arab Emirates and its relation to evaporative dolomite genesis: SEPM Spec. Pub. 28, p. 1 1 -30. Meissner, F. F. , 1 974, Hydrocarbon ac· cumulation in San Andres Formation of Permian Basin, southeast New Mex-
pol ygons and mudcracks. deveioped in hum i d climates. such as at Andros Island in the Bahamas. (C) shows the sedimentary feat'J res associated with arid tidal flats, such as on the Trt.:cia! Coast lr. the Persia n Gulf or in the Shark Bay region , western Aus�ra 11a. Note that the major differP.nce in :he :ast case 1s the presence of nodular or chicken wire anr1ydrite . The intertida; zor.es can range from oxidized muds 'o {0) coral reefs to (G) rio�\ed Cr<'I�Mh?.c1
ico and west Texas : Abs. , AAPG Bull . , v. 5 8 , no. 5, p. 909·9 10. Multer, H. G., and J. E . Hoffmeister, 1 968, Subaerial laminated crusts of the Florida Keys: Geol. Soc. America Bull . , v . 79, p . 1 83-192. Newell , N. D . , and J . K. Rigby, 1957,
lng large clu b-shaped algal structures, sue� R� l hose shm.Yn evrlier in Figu res 1 5A and B. The rigllth a'ld section is an <Jft€mpl to show relative abundance of va rious sedime ntary str uch.Jrcs. grains. minerals ar�d fossils, bas!'!d on tl:e � i teraturc �nd per�onal observ�Hons ot modern o.nd ancient seo.uer.ces. It 5ho�1d be emph asized that r:o geologic exa mple Is Hke;y to lit exactly ali !he features shovm here.
Geologic studies in the Great Bahama Bank , in Regional aspects of carbonate deposition: SEPM Spec. Pub . 5, p. 1 5-79.
Pilkey, 0 . H . , 1 966, Carbonate and clay mineralogy of the Persian Gulf: Deep Sea Research, v. 1 3, p. 1 - 1 6.
21 0
Tidal Flat
Purser, B. H., 1 973, The Persian Gulf Holocene carbonate sedimentation and diagenesis in a shallow epicontinental sea: Heidelberg, Berlin, Springer-Verlag Pub., 471 p. __ and G. Evans, 1973, Regional sed imentation along the Trucial Coast, SE Persian Gulf, in B. H. Purser, ed., The Persian Gulf-Holocene carbonate sedimentation and diagenesis in a shallow epicontinental sea: Heidelberg, Berlin, Springer-Verlag Pub . , p. 21 1 -2 1 3 . __ and J. P. Loreau, 1 973, Aragon itic, supratidal encrustations on the Trucial Coast of the Persian Gulf, in B . H . Purser, ed., The Persian Gulf-Holocene carbonate sedimentation and diagenesis in a shallow epicontinen tal sea: Heidelberg, Berlin , Springer Verlag Pub., p. 343-376. Read, J. F., 1 975, Tidal-flat facies in carbonate cycles, Pillara Formation (Devonian), Canning Basin, Western Australia, in R. N. Ginsberg, ed., Tidal deposits, a casebook of Recent ex amples and fossil counterparts: New York, Springer-Verlag Pub . , p . 25 1 -256. Reinick, H. E., 1967, Layered sediments of tidal flats, beaches, and shelf bot toms of theNorth Sea: Estuaries , Am . Assoc. Advan. Science, v. 83, p. 1 9 1 -206. Robbin, D . M . , and J. J. Stipp, 1979, Depositional rate of laminated soilstone crusts, Florida Keys: Jour. Sed. Petrology, v. 49, no. I , p. 175- 1 80. Roehl, P. 0., 1 967, Stony Mountain (Or dovician) and Interlake (Silurian) facies analogs of Recent low-energy marine and subaerial carbonates, Bahamas: AAPG Bull., v. 5 1 , p. 1 979-2032. Rogers, J. P., 1 97 1 , Tidal sedimentation
and its bearing on reservoir and trap in Permian Phosphoria strata, Cotton wood Creek Field, Big Horn Basin, Wyoming: Mtn. Geol . , v. 8, no. 2, p. 7 1 -80. Schneider, J. F., 1975, Recent tidal depos its, Abu Dhabi, United Arab Emirates, Arabian Gulf, in R. N. Ginsburg, ed. , Tidal deposits, a casebook o f Recent examples and fossil counterparts : New York, Springer-Verlag Pub . , p. 209-214. Scholle, P. A . , and D. J. J . Kinsman, 1974, Aragonitic and high-Mg calcite caliche from the Persian Gulf-a modern analog for the Permian of Texas and New Mexico: Jour. Sed . Petrology, v. 44, no. 3, p. 904-9 1 6. Shinn, E. A., 1 968a, Selective dolomitiza tion of Recent sedimentary structures: Jour. Sed. Petrology, v. 38, no. 2, p. 612-6 16. __ l 968b, Practical significance of birdseye structures in carbonate rocks: Jour. Sed. Petrology, v. 38, no. 1 , p. 21 5-223. __ 1968c, Burrowing in Recent lime sediments of Florida and the Bahamas: Jour. Paleontology, v. 42, no. 4, p. 879-894. __ 1969, Submarine lithification of Holocene carbonate sediments in the Persian Gulf : Sedimentology, v. 12, p. 109-1 44. __ 1 972, Worm and algal-built col umnar stromatolites in the Persian Gulf: Jour. Sed. Petrology, v. 42, no. 4, p. 837-840. __ 1 973a , Carbonate coastal accre tion in an area of longshore transport, NE Qatar, Persian Gulf, in B. H . Purser, ed., The Persian Gulf-Holocene carbonate sedimentation and diagenesis in a shallow epicontinen-
tal sea: Heidelberg, Berlin, Springer Verlag Pub . , p. 1 79- 1 9 1 . __ 1 973b, Sedimentary accretion along the leeward, SE coast of Qatar Peninsula, Persian Gulf, in B. H . Purser, ed . , The Persian Gulf-Holocene carbonate sedimentation and diagenesis in a shallow epicontinen tal sea: Heidelberg, Berlin, Springer Verlag Pub . , p. 1 99-209. __ D . M . Robbin, and R. P. Steinen, 1980, Experimental compaction of lime sediment: Denver, Abs. , AAPG Ann . Mtg. , p. 1 20. __ R. N. Ginsburg, and R. M. Lloyd, 1 965, Recent supratidal dolomite from Andros Island, Bahamas, in L. C. Pray and R. C. Murray, eds . , Dolomitization and limestone diagenesis-a symposium: SEPM Spec. Pub. 1 3 , p. 1 1 2- 1 23 . __ R . M . Lloyd and R . N . Ginsburg, 1 969, Anatomy of a modern carbonate tidal flat, Andros Island, Bahamas: Jour. Sed. Petrology, v . 39, p. 1 202- 1 228. Steinen, R. P . , and R. B. Halley, 1 979, Ground water observations on small carbonate isl an ds of southern Florida, in R. B. Halley, ed., Guidebook to sedimentation for the Dry Tortugas: Southeastern Geol. Soc. Pub. No. 2 1 , p . 82-89. Tebbutt, G. E . , C. D. Conley, and D. W. Boyd, 1 965, Lithogenesis of a distinc tive carbonate rock fabric, in R. B. Parker, ed . , Contributions to geology: Laramie, Univ. Wyoming, p. 1 - 1 3 . Van Straaten, L . M. J . U . , 1 954, Sedi mentology of Recent tidal flat deposits and the Psammites du Condroz (Devo nian): Geol. Mijnbouw, v. 1 6 , p. 25-47. Wells, A. , 1 962, Primary dolomitization in Persian Gulf:Nature, v. 1 94, no. 4825, p . 274-275 .
21 2
Beach
F ig u re 1 -Diagram showing the varia· tion in beach development along the Trucial Coast of the Persian Gulf (after Purser and Evans. 1 973).
M
ost ancient carbonate those composed of carbonate grains strata were deposited on are fed by a nearby offshore source extensive, warm, shaJ of sediment that is being formed low, marine shelves, and through physiochemical precipitation record regressive rather than trans (oolites , grapestones), the breakdown gressive periods of sedimentation. of carbonate skeletal material, or the erosion of previously deposited lime These sequences commonly contain significant volumes of rocks deposited mud or submarine cemented layers (intraclasts); siliciclastic beaches are, in low energy, shallow, subtidal-to for the most part, fed directly by supratidal, strandline lithotopes. At the same time, however, higher energy streams or by long shore currents car rying stream-derived sediment . wave-dominated, strandline deposits We suspect that many ancient (for example, beaches) are surpris bioclastic and oolitic calcarenite shoal ingly scarce in the geological record. deposits are comprised in part of We feel that this is probably due to unidentified beach lithofacies, and lack of recognition, rather than an that many of the thin fossiliferous, actual absence of beach deposits in oolitic, and intraclastic calcarenite carbonate strandline sequences. zones found as interbeds between Inasmuch as shallow marine evaporites, laminated calcilutites and siliciclastic sequences frequently con dololutites , and other limestones, are tain both low energy tidal flat and also beach deposits. Susceptibility of higher energy beach units along their shorelines, we should expect the same the grains in these beaches to the syn diagenetic and early burial processes to be true in many carbonate se of cementation or exposure of car quences , because energy flux varies along strandlines in response to basin bonate beach sediment to weathering stabilization processes (recrystaJliza hydrology and topography, irrespec tive of the mineralogy of the sea floor tion , cementation) commonly results sediment. The only real difference be in their lithification. The strandline tween the two strandline types is that sequence is thus rendered more resis-
tant to mechanical erosive processes during erosional transgressive episodes, or scour by tidal, alluvial, or distributary channels in laterally equivalent and overlying facies . Its chance of preservation , therefore, is greater than in equivalent siliciclastic systems. DEFINITION
Beaches are wave-dominated coastal sedimentological systems com posed of loose sediment , whose overall external morphology and in ternal character may contain modifications induced by tidal activi ty and/or longshore currents . Their upper and lower environmentally defined limits are thus set where wave current activity is no longer domi nant. Laterally, they can grade into tidal flats because of a decrease in the availability of sand or coarser grain sizes, and/or a change from wave- to tidally-dominated processes along the coastline . Beach surfaces are inclined seaward at angles steeper than those characteristic of tidal flats because of the combined actions of intense tur-
Beach
Z.ON E S
213
(B'ofore Bvrlal) �� �
� R� �� · ����--�4-�����������������������--��� Smoll
FOR ESHORE
Scol& Ava lane��
Cross !'!ddi ng
Fine Gf
Wave 5waoh
VEr,i.rol Sunows
SHOREFACE
OfF SHOR E
\
Gr<:�i nston� "'
Pockstono Poekston� to Wockeston-:
Sr::n o ll to Medium Cro"b�s
Hcxi:r:onlol Bronch�g BurrQW:s
Dire
Biologic
Figure 2-Generallzed beach model il lustrat i ng the most simple and common beach setting. Conceptualized after Ber
nard and Majors (1 962).
bulence in the wave breaker zone at the toe of the beach, and the pro gressive loss of energy up the beach toward the upper limit of the swash zone. Finer grained sediment is win nowed away by incessant wave ac tion, and, even if it were not, it could not stand at a steep enough angle of repose to be recognized as a beach if one were standing on it . The width and steepness of the active beach zone being affected by daily wave ac-
tion is dependent upon wave size, as well as tidal activity (tidal height and current strength) and the size of con stituent grains. The general environ ment includes barrier islands, spits, and non-barrier mainland beaches, as well as isolated island beaches (Fig. 1 ) . These isolated beaches may bear some relationship to the regional coastline, such as those associated with paralleling reef systems or horst blocks, or they may develop random-
ly in association with salt domes, mud mounds, patch reefs or drowned topographic features. Variations in beach settings and their internal and geometric characteristics will be presented in the following pages. General Model
The basic process-response model for carbonate beaches generally parallels the clastic barrier island model proposed by Bernard and
21 4
Beach
• . ;.·f�
�
.
: ....����t ;�:�
Fig u re 3a-Piei stocene beach ridges
showing seaward dipping carbonate sa nd rich accretion layers; South African coastline (ph oto by Dave Hobday).
Figure 3b-Seaward dipping beach l i me
others ( 1962) for beaches that develop grainstone accretion beds; Edwards For in a microtidal (0 to 2 m) setting. mation, Round Mountai n , Texas . Since then, many sedimentologists working in numerous other (especially clastic) beach settings (Clifton et al, 1971; Hayes, 1976; Ball, 1967 ; Hub bard et al, 1 979; Shinn, 1973; Kumar and Sanders, 1974) have added to our knowledge of how this basic model varies in response to different wave, longshore current, and tidal energy regimes. It is not our intention to describe all the sedimentary structures and structure sequences that result from varying grain size, and the types and amounts of energy input to strandlines. Instead, we will keynote Figure 3c-Arcuate beach ridges: Joulters Cay, Bahamas (courtesy Mitch those structures and sequences Harris)� necessary for beach definition and recognition. The beach depositional environ ment, exclusive of the backshore zone, represents, in its simplest form, a seaward-inclined depositional inter dunes, washovers, and supratidal complexes which form as a result of face along which the high energy wind, storm, and tidal action. These regimen found in the surf zone gradually decreases in a seaward grade laterally landward into (deeper water) direction. This inter lagoonal, sabkha, or continental set face may be divided into three grada tings. A soil profile may separate the tional subzones (Fig. 2), identified by main beach from the low energy backshore deposits, or it may develop the dominant depositional processes operative in each zone. The foreshore across both beach and backshore sediments. In a vertical sequence is dominated by high energy wave representing one regressive cycle, swash and related processes. The backshore units lie directly above the shoreface is the zone in which longshore and tidal current activity is main beach deposits, which in turn grade downward into finer-grained pronounced, and the offshore en offshore units. The process-product vironment is a lower energy environ relationships responsible for this char ment , where biological processes, acteristic sequence are outlined in such as bioturbation, predominate. Figure 2. The backshore environment includes
Vertical Textural Sequence
The main textural response to the energy flux across the beach zone is the removal of mud from the sedi ment, and the gradation from well winnowed sands (lime grainstones) in the upper foreshore to progressively muddier sediments (lime packstones and wackestones) toward deeper , lower energy environments. Foreshore units most often have a small percent age of coarser grains mixed in with a dominant finer, well sorted grain population (negatively skewed), but they are also commonly very well or well sorted (Folk and Cetera, 1970). In general , size and sorting decrease vertically down the depositional inter face, as illustrated in Figure 2. However, the coarsest sediment may occur in the middle of the interface (Folk and Robels, 1964; Folk and Cotera, 1 970), which marks the tran sition zone between the lower foreshore and upper shoreface. The grain sizes present, as well as their composition, are in part a func tion of wave intensity and current strength acting on the beach inter face. To a large extent , however, these parameters are controlled by the variety and individual numbers of shell-bearing organisms living in the beach and in near offshore en vironments. This is true because these organisms are the primary sediment contributors to the beach, and each type has a shell which breaks down at
Beach
215
Figure 4-Uppermost portion of beach accretion beds showing gentle seaward dip (right) and u pp e rmost bed dip rever sal toward backshore (left). Cow Creek Limestone, Lower Cretaceous. Texas.
Fig u re Sa-Gently seaward dipping eve nly lam i nated sequences in upper· most part of beach accretion unit; South Carolina coast (courtesy John Barwis).
a specific rate into preferred grain sizes . Grain roundness is also a func tion of energy input to the sediment as well as its composition, and in general, i� will decrease downward in to lower energy environments. This, of course, would not be the case if oolite shoals were the source of the beach sediment.
Figure 5b-Laminated foreshore beds, beach sequence in M ississippian Newman Limestone, Eastern Kentucky.
216
Beach
Figure 6a-Large vertical and subver tical bu rrows penetrating even, para l lel laminations i n upper part of beach ac cretion bed, Pleistocene, South Africa (photo cou rtesy of Dave Hobday).
Figure 6b-Worm burrows in coarse grained mollusk l i m e grainstone of Lower Cretaceous beach foreshore u nit s .
Figure ?a-Air-bubble holes i n upper· most part of swash zone; holes are ap prox imately 3 to 5 mm in diameter.
Fi g u re 7b- Large i rregu l ar keystone vugs in slab of Recent carbonate beachrock, Grand Cayman, British West I ndies.
contains all microfacies of the beach foreshore and shoreface and fade into typically lower energy offshore facies. The most obvious sedimentary structures in outcrops of ancient At the top of the sequence, it is com mon to see berm deposits, with dip beaches are gentle, seaward-dipping (5 to 1 5 °), large-scale planar accretion reversal toward the mainland (Fig . 4), that are associated with overlying and beds (Fig. 3a, b) which record the infilling carbonate muds. progradation of the beach over Within each progradational accre deeper, subtidal sediments. These tion unit there is a distinct suite o f units are associated with the beach ridges that mark prograding Holocene smaller-scale sedimentary structures beaches (Fig. 3c). Each accretion unit which reflect the changing energy Sedimentary Features Physical and Biological
regimes across the depositional inter face of the beach. The uppermost part of the sequence represents deposition by wave swash processes in the beach foreshore zone (Fig. 2) and includes alternating fine- and coarse grained , sometimes inversely graded , even laminations which have a gentle seaward dip (Fig . 5). A wide variety of vertical and subvertical mollusk, worm, and crustacean burrows, with morphologies characteristic of high
Beach
217
ovoid keystone vugs in Recent carbonate beachrock; sca!e Figure 7c-Large
equals 1 .0 mm.
Figure 7d-Keystone vug Clower left cor· ner}. in oolitic l ime grainstone; M i ssissip· pian Newman Formation, Kentucky; scale equals .3 mm.
energy, high sedimentation environ ments, penetrate these laminations (Fig. 6a, b). The uppermost lamina tions of the beach accretion beds often contain tabular to somewhat spherical cavities referred to as keystone vugs (Dunham , 1 970; Fig. 7) . These represent voids left by air and gas bubbles, which form im mediately above the wave swash zone as a result of air escaping from in-
tergranular pores as they are flooded with marine waters during the flood tidal cycle. Oscillation and current ripples, as well as current lineations and antidunes, are present on accre tion bedding planes in the laminated swash zone, but these are rarely observed in ancient sequences because of their modification into upper flow regime flat bedforms (Harms, 1 979) before burial.
A zone in the center of the vertical sequence and j ust below the swash laminations (Fig. 2) , is referred to as the shoreface, and is dominated by small- to large-scale tabular and fes toon crossbeds (Fig. 8a, b) . This zone represents deposition by currents flowing predominantly parallel to the beach (Fig. 8c) , and thus most cross bed dip directions are oriented perpendicular to the beach accretion
218
Beach
Figure 8a L-� rge 1et\IO<Jn (irough) cross bcdrs irnmedi�taty below beach foreshore ltnit (upper lalt of photo). Ori�::n tation of crossbads indicates tlow parall�l �o d�sitional stn�..e of beach, Lower Cretace<x.ts r:� CrMk limP-Stone Texas.
Figure 8b-CtosNe-:;tional view of large ·fesioon (trough) c rossbi?!ds und�r !aminai�d b�ar;h tc.rea hora saquenCP.; So•Jth African coos tiiM (Pharo courtes�r of Dave Hoooay).
FigL1re Sc-Tidal channel cutting lhrO\..Igil Re<Xi!nt beach systems. Joul1cr·s Ca.� ·
Ba.hlO!mas.
Beach
21 9
Figure 9a-Branching crustacean bur
row fill from a low energy subtidal
Florida Bay envi ronment. Burrow fills may be finer or coarser grained than the t!urrounding matrix sedi ment, depending mostly on what the organism i ngested, and what grain-size sediment the overly Ing unit Is composed of.
Figure 9b-Cru stacean bu rrows In shallow subtidal deposits beneath beach shoreface deposits. Burrow fills are slightly arenaceous pellet lime/packstones, whereas the matrix rock is a fossilirerous, dolomitic quartz sandstone, Lower Cretaceous Cow Creek Limestone, Texas.
beds (Stricklin and Smith, 1 972). Landward oriented planar crossbeds may also be present , in which case the system probably represents the preserved ridge and runnel zone that commonly develops in mesotidal (2 to 4 m) beach settings (Hayes, 1 976) . Crossbeds may be composed of sedi ment coarser or finer grained than the overlying laminated zone . They have either a sharp scour contact with , or grade downward into, the offshore
deposits, and may be interbedded with the lowermost laminated foreshore deposits. Underlying offshore deposits are typically composed of bioturbated and burrowed lime (or dolomite) packstones and wackestones . Distinct burrow traces within these units are either horizontal or branching, and reflect the prevailing lower energy conditions in offshore areas (Fig. 9a, b) . Thin, rippled lime packstone and
grainstone interbeds and lenses (Fig. 9d), commonly with sharply scoured lower contacts and bioturbated or bored upper contacts, are often pres ent immediately below the shoreface zone; they represent storm deposits formed through the winnowing of the underlying and laterally equivalent sediments. The biota contained within the offshore deposits may be highly varied and abundant or extremely limited , depending on environmental
220
Beach
Figu re 9c-Subhorizontal Thal/asinoides (crustacean) burrows, shallow offshore
facies, Lower Cretaceous Cow Creek Limestone, Texas.
Fig u re 9d-Thin ripple c rosslaminated lime packstone lenses with basal scour contacts into finer grained shallow off. shore facies, Lower Cretaceous Cow Creek Limestone, Texas.
Figure 1 0a-Storm wa s hovers beh i n d a long straight barrier beach. Sediment from the beach zone and offs hore en v i ronments is dispe rsed i nto the backshore zone via storm s urg e ch a n n e l s {arrow ). Large w as h ov e r i n foreground is now stable a n d h a s a com plex of subenvironments {supratidal marsh and pond, t i da l creeks, eol i an dunes and sand fla ts ) d e ve l oped on it (photo cou rtesy M. 0. Hayes).
conditions. In either case, however, it will be similar to the biota found in the foreshore and shoreface sediments. The backshore environment is made up of a myriad of sedimentary subenvironments because of the numerous processes that act in its for mation . In the geologic record , how ever, only storm washovers and thin tidal flat and supratidal units are common. Dune deposits, although
present in the Holocene and Pleisto cene, seem to be rarely preserved or recognized in older units. Eolian dunes form as a result of strong landward directed winds eroding sediment from the foreshore and depositing it in the immediate backshore area. These dunes grade laterally into progressively finer grain ed sand flats toward the far backshore areas. Sediment in these sand flats is derived from both the dunes and the uppermost layers of washover fans. Washover fans are deposited by wave induced currents carrying sedi ment through channels cut into low lying beach areas; they form during intense storm activity and can make up the bulk of the backshore sedi ment volume. Fan deposits cover from a fraction to 4 sq km of backbeach supratidal marsh , intertidal flat, or lagoonal floor (Fig . l Oa, b). They are primarily composed of evenly laminated grainstones and packstones . Normal ly, they are thin ( . 1 to 1 m), become finer-grained upward, and have scoured or sharp lower contacts. Basal fossil and intraclast-rich lags, as well as avalanche (tabular) style cross bedding deposits, are common in the proximal and distal portions of fans, respectively (Fig. l Ob; Schwartz,
Beach
221
Figure 1 Ob-Washover fans prograd ing {from right to l eft) onto algal·matted. low energy tidal flat; steep frontal marg i n of washover indicates i nternal avalanche style planar crossbedd ing .
Figure 1 1 a-Seaward dipping slabs of Recent beachrock cemented foreshore sediment, Grand Cayman Island, British West Indies.
1 975). Large branching crustacean burrows, smaller vertical "pencil" burrows, and roots typically penetrate their upper layers . Their distal por tions, and areas between them, may be dominated by tidal flats or lagoonal deposits, or they may grade landward directly into sabkha or con tinental settings. Supratidal and/or intertidal lithologies are highly varied in character (Shinn, 1982), and supratidal sequences generally make up the bulk of the beach backshore sequence in ancient carbonate se quences . Sedimentary Features - Diagenetic
Carbonate beach deposits are sub ject to certain diagenetic processes soon after their deposition . The resul tant diagenetic products are somewhat unique to the beach set ting, and are thus useful as criteria for beach recognition . The most com mon diagenetic process is penecontemporaneous beach cementa tion associated with the foreshore en vironment. The most obvious product of this early cementation is the development of large seaward dipping slabs of beachrock in the foreshore zone (Fig. l l a). During storms these slabs are undercut by wave and cur rent action, causing collapse, breakage and reworking into tabular
cobble to pebble size beachrock clasts (Fig. l lb , c). These clasts are recognized in ancient beach sequences by subtle textural , cementation , struc tural (disoriented laminations) and color contrasts with the surrounding beach deposits (Fig. l l c). On a smaller scale, the margins of the clasts display truncated grains and cements (Fig. l ld), and may be penetrated by the borings of various
marine organisms (worms, clams, barnacles, sponges, etc.). Donaldson and Ricketts ( 1 979) describe in detail the occurrence of beachrock clasts in a beach sequence from the Precam brian of Canada. The waters from which beachrock cements are precipitated are generally marine, so that the mineralogy of the cements is dominantly aragonite, and occasionally high-magnesium calcite.
222
Beach
Figure 1 1 b-Large collapsed beachrock
slabs, probably due to wave undermining in l am inate d beach 1oreshore beds; Lower C retaceous Edwards Limestone, Texas.
Figure 1 1 c-Rounded cobble and boulder size slabs of beachrock contrast in color, texture, and structure witl1 sur· rounding beach lime g rainstones; Lower Cretaceous Edwards Limestone, Texas.
Figure 1 1 d-Beach rock clast displaying truncated g rains and isopachous,
forme rly aragonite{?) beachrock cement;
Lower Cretaceous Cow Creek (scale equals .3 mm).
Limestone, T exas
Aragonite beachrock cements usually occur as thick circumgranular acicular crusts which sometimes occlude all pore space (Fig. 1 2a, b). High magnesium calcite cements are pres ent as thin-bladed isopachous crusts (Fig. 1 2c) or, more commonly, as dark or golden brown, pelletal and micritic, irregular grain coatings and void fillings (Fig . 1 2d , e ; Alex andersson , 1 972). The latter often contains abundant microfossils and
may form, at least in part , as a result of intergranular biological activity (Moore, 1 973) . These cements may exhibit microstalactitic and meniscus orientation fabrics (Fig. 1 2e , f, g) , both of which are unique to the vadose zone if they form in the inter tidal or supratidal portion of the beach . Below low tide level, acicular cements typically occur as somewhat regular or isopachous crusts (Fig. 1 2c) . These cements, because of their
mineralogical instability, may be dissolved or recrystallized during later diagenesis, and thus , are not always recognizable or present in ancient beach sequences . However , their " fossilized" occurrence in ancient beach sequences has been well documented (Petta, 1 977a; Moore et al, 1 972) and thus provides us with an additional set of criteria by which we may recognize ancient beaches . Because of the slow rate of deposi-
Beach
223
Figure 1 2a-Aragonlte needle fringe cements rimming algal grains; note slight irregularity in thickness of cement; Grand Cayman Island, British West I n· dies {scale equals .25 mm).
Figure 1 2b-Fibrous, somewhat ir· regular iringe cements, originally thought to be a ragonite, from M ississip· pian beach sequence in Kentucky (scale equals .25 mm).
tion on many beaches, a variety of soil features may develop on and in the foreshore units, finer grained backshore supratidal facies, and washover deposits. The soil forming processes normally lead to grain dissolution, microfracturing, micritization, and general destruction of the original texture and fabric of the deposits (Fig. l 3a, b). Diagenetic fabrics associated with soil formation, such as vadose cements, pisolites, and
coated grains occur in the upper 1 or 2 m of the foreshore grainstones and backshore units (Fig. 1 3c, d) . Nodular caliches develop out of, and on top of, these beach deposits, and may be separated from them by a hummocky iron-stained dissolution surface (Fig. 1 3e ; Inden, 1 972). Laminated subaerial crusts (Fig. l 3 f, g; Multer and Hoffmeister, 1 968) coat complexly brecciated black and brown micritized clasts and many ex-
tend downward along previously formed fractures that cut across and parallel foreshore bedding units (Ferm et al, 1 97 1 ; Inden and Horne, 1 973; Fig. 1 3h). Beach Recognition
Identification of beach sequences rests on the recognition of the vertical mosaic of textures , fabrics, and sedimentary structures which together indicate an overall downward grada-
224
Beach
Figure 1 2c-H igh-magnesium calcite isopachous fringe cements; note how regu l ar this ce ment development is com pared to Figure 1 3a; Grand Cayman Island, British West I ndies; scale equals . 1 25 mm (stained with Clayton Yellow).
Figure 1 2d-H igh-magnesium calcite micritic and pe ll etal cements; If cemen· tation of this type goes to completion the sediment wi ll lose its original textural Identity of being a lime grainstone and will be converted diagenetically Into a lime packstone; Grand Cayman Island, British West I ndies (scale equals . 1 25 mm}.
tion from high energy (upper flow regime) environmental conditions to normally lower energy environmental conditions. Evidence of exposure, such as fine-grained supratidal car bonates and/or evaporites, paleosols, or subaerial crusts, must be present above the characteristic high energy, low angle, evenly laminated foreshore lime/ grainstone beds in regressive
beach sequences . Unique sedimentary features, such as keystone vugs, reworked beachrock clasts, and marine vadose beachrock cements, are common and are generally useful in the recognition of ancient beach sequences .
GEOLOGIC SETTINGS, ASSOCIATED FACIES, AND BASIC BEACH MODEL VARIATIONS
Variations on the basic beach model (Figs. 2, 1 4a) are controlled by the geomorphic and sedimentologic setting of the beach . Major variations are formed when the beach : (1) is at-
Beach
Figure 1 2e-Very regular isopachous c rusts coated by mic rite cements displaying meniscus and microstalactitic fab r ics , both of which are characteristic of precipitation in the vadose zone: scale equals .25 mm; Ju rassic (Dogger) of Great Brita i n .
tached to a mainland, (2) forms a barrier with potential tidal inlets and a low energy lagoonal environment on its landward side, (3) has high energy (rather than low energy) off shore equivalent facies , such as a reef or bioclastic-oolitic sand sheet or shoal complex; or (4) develops in a low energy environment . Mainland Beaches
Mainland beaches form regional strandlines along continents. or large islands. Landward facies equivalents are carbonate, evaporite (sabkha) , or clastic (coastal plain, alluvial fan) se quences depending on climate and the relative deliverability rate of offshore derived marine carbonate sediments , and landward-derived siliciclastic detritus. A Holocene example is the regional beach system along the northeastern Trucial Coast area of the Persian Gulf (Fig. 1 ; Purser and Evans, 1 973). The coastline in this region is exposed to the deep, open waters of the Persian Gul f, as well as being oriented at almost right angles to ' 'shamal ' ' winds which blow across the width of the gulf from a west-
Figure 1 2f-Mic rostalactitic beachrock cements (arrows) d eve lo ped in a very coarse algal coated grain lime grainstone, Lower Cretaceous Stuart Ci ty Reef Trend, Texas (scale in mm).
northwesterly direction. Thus, without the baffling effect of shallow barriers, such as reefs or other struc tural and topographic controls, the unimpeded waves generated in the open gulf hit the coast with maximum fetch, and in the process generate strong longshore currents. Because of the continuous and rapid production of bioclastic debris in the offshore area, the mainland beach sequence has prograded from 5 to 10 km seaward during the Holocene (Purser and Evans, 1 973). This has resulted in the development of a sheet of sand which is elongate parallel to the coast and up to 1 5 m thick and 10 km wide. It is underlain by bioturbated, finer grained, poorly sorted molluscan skeletal sands (lime packstones and wackestones). If pro gradation continues, it will eventually be overlain by sabkha deposits and distal alluvial fan muds and channels sands. A mainland beach also exists on the western Trucial coast (Fig. 1 ; Purser and Evans, 1 973), but here the composition o f the beach sands varies tremendously from east (coral-algal) ,
225
Figu re 1 2g-Microstalactitic cement
(MS), (probably originally a ragon ite) displaying g rowth lines of (o r i gin a l l y high magnesium calcite) micrite (a rrow); micrite below (m) may be infiltrated sedi ment or cement; scale equals .065 mm; Lower Cretaceous, Texas.
where reefs exist offshore , to west (oolite), where the beach system is in creasingly influenced by the wind shadow created by the Qatar Penin sula. Offshore deposits are rippled and crossbedded at shallow depths ( 1 t o 2 m ) and rippled, grading to bioturbated, at greater depths (Purser and Evans, 1 973) . The Cow Creek Limestone and equivalents serve as an ancient exam ple and will be treated in the last sec tion of this chapter. Barriers
Barrier beach sequences are the most common variant of the generalized beach model, and show the widest variety of associated facies. The major additional facies com ponents are those deposits that form as a result of current activity through tidal inlets which pass through the barrier into the back barrier lagoon (Fig. 1 5a, b) . These processes lead to the development of a downward coarsening sequence which is as thick or thicker (up to 12 m) than the laterally equivalent shoreface and foreshore units. The result is the for-
226
Beach
Figure 1 3a-Holocene beach lime g rainstone being altered i n to soil (caliche) through processes of grain micritization and precipitation of micrite cem�nt; small vugs are also forming as a result of dissolution: St. Croix. Virgi n
Islands (scale equals .5 mm).
Figure 1 3b-Paleocaliche from top of beach sequence. Original lithology was a mollusk lime grainstone, but precipitation of micrite cement, intense micritization, dissolution, a n d fracturing have destroyed its original tc>cture and fabric and converted it into a lime wackestone; Lowe r Cretaceous Caw Creek Li m esto n e , Texas (scale equals .25 mm).
mation of a couplet consisting of two high energy end components which may be separated by somewhat lower energy lime wackestones and packstones (Fig. 14b). Because tidal inlets may migrate along the strike of the barrier, the following normal progression of sedimentary features is commonly observed in Holocene as well as an cient carbonate and clastic barrier complexes (Boutte, 1969; Hayes,
1 976; Hubbard et al, 1979) deposited under a microtidal (0 to 2 m) or mesotidal (2 to 4 m) setting. The inlet sequence itself is bounded by a basal scour surface (Figs. l 4b , l 5c) which may have associated coarse intraclast or fossil lags immediately above it (Fig. 14b). These are overlain by large- and medium-scale tabular crossbedded lime grainstones (Fig. l 6c, d), which in turn grade upward into small scale tabular and festoon
crossbedded, as well as ripple cross laminated, deposits. Crossbed dip directions are typically unimodal in the upper and lower part of the se quence and trend perpendicular to the strike of the barrier . Ebb-oriented crossbeds typically occur in the lower part of the inlet fill. Wave-produced flood (landward) oriented crossbeds may well dominate its upper portions, but bimodal crossbeds are common in this zone as well . The sequence pro-
Beach
227
Figure 1 3c-Microstal actitic and in upper part of beach and immediately below l ithology
dolomicrite cements
In photo 1 4b; Lower Cretaceous Cow Creek Limestone, Texas (scale equals .25 mm).
FigurE:1 1 3d-Micrite coaled grains, micrite C€ments and microstalactitic micrite cement (arrow) in
pelletal
paleosol capping M ississippian oolite shoal sequence, Kentucky (scale equals . 1 25
gressively fines upward into offshore, lagoonal , or backshore tidal flat and supratidal facies, or is capped by coarsening upward shoreface and foreshore deposits, either abruptly, or with a thin intervening offshore unit (Fig . 1 6e). Tidal delta and tidal inlet fills are rare in microtidal barrier island deposits (Fig. 1 5a), but nor mally make up a large part of the mesotidal barrier sequence (Fig. 1 5b; Hayes, 1 976).
Barriers are best developed in the Persian Gulf along the central Trucial Coast (Fig. l ; Purser and Evans, 1973). A structural high, referred to as the Great Pearl Bank, gave rise to an offshore topographic ridge prior to the onset of Holocene develop ment; an intervening topographic low (now the Kor al Bazm lagoon) separated this ridge from the mainland. At the onset of the Holocene depositional cycle, reefs
mm).
began to grow on the crest of the ridge, and coral-algal skeletal sands began accumulating around them to form small subaerially exposed islands (Fig. 1 6a). These islands began to expand lengthwise along the trend of the ridge because of con tinued lateral accretion due to long shore drift (Fig. 16b). Eventually, tidal flat areas began to form in the protected areas behind the islands, while channels in the topographic low
228
Beach
Fig ure 1 3e-Hummocky i ron-stained dissolution su rface separating wel l ·
developed nodular caliche (N} from underlying partially callchlfled beach
lime g rainstones; lower Cretaceous Cow Creek Limestone, Texas.
Figure 1 3f-Su rface of Pleistocene iron stained subaerial crust, Florida Keys.
behind the ridge progressively became more restricted (Fig. 1 6c) . Tidal deltas and tidal inlet fills of bioclastic-oolite sands began to form , and the Kor al Bazm lagoon started to fill with low energy pelletal muds and tidal flat deposits (Purser and Evans, 1 973; Fig. 1 6d). If the present system continues to operate, the lagoon will become a highly restricted environment and subaqueous
evaporites will begin to precipitate. The entire area will eventually be con verted into, and capped by, a low energy , evaporitic, tidal flat-sabkha system . This portion of the Persian Gulf, and its depositional evolution , serves as a n excellent Holocene analog for the Mission Canyon For mation, which is discussed in the lat ter part of this chapter .
Beaches Associated with Higher Energy Shelf Environments and Reefs
Often in carbonate provinces, the shelf is a high energy environment. A shoreline adjacent to such a shelf will not exhibit the normal coarse to fine grained downward transition. Instead, beach foreshore and shoreface deposits will be underlain by oolitic or bioclastic marine sand sheets or
Beach
-1
:;
!?
,
,
G
·�
1 0
.1• .: .
� �
1 3
:. .r -
1s
1· o
1· 1
229
Figure 1 3g-Mississippian SIJbaerial c rust displaying blacken m i c rite clasts, multiple frac t u r ing and in situ breccia tion, pisolites, and micrite coatings (light colored) on black m i c rite clasts (scale in
mm).
Figure 1 3h-Subaerial c rust develop ment along bedding plane and c ross cutting fractures in Mississippian
(Newman Formation) of Kentucky.
shoal units (Fig. 17a, c) similar to those that form as marine sand belts and tidal bar belts near the Bahamian shelf margin (Ball , 1967). Hine (1977) and Harris (1979) describe in detail the development of these systems, their internal characteristics, and the morphology of the subtidal sand bodies which this beach type even tually caps. Beach-dominated islands also form in reefal settings, such as around patch reefs on broad shallow shelves and depositional ramps (Folk and Robles, 1964; Purser , 1973; Flood, 1974; Petta, 1977a, b) or along shelf
margin barrier systems (Bebout and Loucks, 1974). This depositional motif existed at the onset of Holocene deposition along the central Trucial Coast (see preceding barrier island discussion), and still exists far ther to the west (Fig. 1) where islands are developing around reefs fringing salt domes or other structural highs (Purser and Evans, 1973). The sediments making up and surrounding these beach islands are inevitably composed of coral-algal rich sands, reflecting their derivation from the reefs upon which the beaches have developed. The beach sediments even-
tually grade into a variety of deeper water, finer grained sediments (for example, open marine mollusk lime packstones, lagoonal pellet-foram lime wackestones, or tidal bar oolite lime packstones-grainstones), depend ing on which environments flank the reefs (Purser, 1973). However, in areas where bottom currents were swift enough to remove most bottom sediment from the area while the reef grew to near sea level , foreshore deposits may rest directly on lithified reef rock or be separated from it by a thin, high-energy crossbedded sand or rubble zone (Figs. 14d, 17c , d, e).
230
Beach
BEAC:H MOOEL VARIATIONS b. "Notmal"
SarrH.r-
lntel"n'1ed1ote
Tidal lnlet
�ergy Offshore
fcot
d. High �ne-rgy
Offshore
9-t&et)
(oolite/blo.-Jas-l:o.
shool. 5nnd
f.
High £net9y Off�ore
(r<:ot:ol) _
•· Tr..,nsgMulve-.
1rotJ�g�ive
(and boclcsha� wosho.,ers)
Regr"(r$sive
m �
.:J
201
30 ••
���� , __.. L .___, I f:Ja I I I 1�1 "----' '--"
Figure 1 4-Variations in general beach model that develop as a result of varying the overall depositional environment in which beaches occur.
Low Energy Beaches
Large waves, and thus effective longshore currents, commonly do not develop in lagoons or restricted marine shelves either because of limited fetch , or great loss of wave energy due to frictional drag along a shallow sea floor. The resulting beaches are formed with thin foreshore units (0. 1 to 1 . 5 m thick; Fig. 1 7e) which grade directly into bioturbated fine-grained liine or dolomite wackestones-mudstones (Fig. 14e) that commonly contain beds or nodules of evaporites. These carbonate units will carry a very restricted fauna, and in many cases the foreshore beds will be com posed of intraclasts , irregularly shaped ooids and pisoids, or the slightly abraded shells of only one species (for example, the gastropod Battillaria almost exclusively makes up the beaches developed in Florida Bay) . These beaches would be ex pected to evolve into, and be overlain by, low energy tidal flat and sabkha deposits . They would form thin lenses
�
Accret ion �ds
festoon Crossb.ds
Bioturbation
-
Shell Fr<>gm••ts
Tabular Crossbed1
lntrodast log
Ripples
Lorninottons & Mudcrac:h
Scour Surface
·��-�
which parallel depositional strike, or would be randomly distributed around the mud mounds that developed in the lagoon . Preservability and Morphologies of Fossilized Beach Facies
Beach complexes may form signifi cant hydrocarbon reservoirs . It is for this reason that it is important to understand which parts of a beach complex might be preserved under different burial conditions. Beach facies stand the best chance of being retained for burial during regressive phases of sedimentation, because deposits of dominantly low energy systems, such as those of lagoonal , supratidal, aeolian, or alluvial floodplain environments, prograde over each lithofacies. Deep channels associated with prograding alluvial or deltaic systems might cut through the underlying beach in places, but for the most part , the beach would be left intact . Regressive barrier island sand bodies and mainland beach sands
(yeo Lu•••irlt,lliuns
....,...
-
Vertical �urrows
R...l Gtainstone
eootsening
llpward
Packstone-Wackestone Pod:,mne- W
t
Fining Upward
Grainstone
Supmtitttl
would be predominantly elongate and aligned parallel to depositional strike; these units might have lengths of up to 5 to 50 km with widths of greater than 70 km (see Cretaceous example of strandline beach) in the case of long progradational histories over stable platforms. Most regressive beach sequences , however , tend to be relatively narrow and lenticular along depositional dip sections. In either case, they pinch out landward into lagoonal, sabkha, or continental units, and marineward into lithologies deposited in deeper, lower energy marine environments . The thickness of barrier sequences might reach 1 5 to 20 m in areas where tidal inlets and deltas formed, but would be less in areas not affected by tidal inlet migration. Tidal inlet sand bodies would also be somewhat pod-shaped perpendicular to the trend of the barrier. Transgression occurs when the rate of sediment supply to the beach system is much lower than the rate of subsidence or rise in sea level. The
Beach
231
depo' .>itional motif showing oH:>hme (a, upper photo), �tJtJ tidal oe1ta (e), flood tidal delta (f), 1fdl:lJ channel (t). anCI lagoonal (b, low�r photo) depot�itivn<�l s�!tingr;. Tldes In off. thore phcto <�re micmtidal. wheree.s In 1he lagoonal photo, tidal activity is in· tei'\Se. figure 1 5a,I>-Barrier island
232
Beach
Figure 1 5c-8asal scour surface of tidal ctwlnel in M isSi$$ ippian (NeW!'l'lan Umestor.e) barrier sequenc e , Eastern Kentucky. Unit OV!$t1yl"9 scextr su rface is ,.es!OOll crossbedd&<j coarsei:Jrajned In· traciasl lime grait1stone.
Figure 1 5d-Large fe.stoon crossbeds at base of 1idal chatlr.el; Lower Cretaceous Edwards Limestone, Texas.
Figure 1 5�-oepos!Mnar ·�uplet displ�ying basal tidal crranrfe l f i l l "unit. (c} fining up.vard Into dark colored orfshore facies (o), which in turn is cver!ait'l by prograding �each accretion units {b). ·
Beach
SEDIMENT/ROCK TYPE
ENVIRONMENT
Coral-Algal lime Boundstone
Reef
Bioclastic lime Gl'oinstones (Coral-algal, Molluskan)
Beach/Sand Flats Around
Oolite Lime Grainstone
Reefs
Beech/Tidal Delta
Mollusk Lime Packstone/ Wackestone
Unrestricted Lagoonlndol Channel Fill
Gastropod-pellet
Restricted Lagoon
Laminated Algal Stroma tolite Dolomite
Tidal Flat
Chickenwire Anhydrite
Sabkha
Lime Packstone/Wackestone end
Mudstone/ Wackestone 8edrock end Continental Gypsum/Dolomite
Figure 1 6-Development of barrier island-lagoonal complex t h rough the Holocene along the central Trucial Coast. Evolution of system shows how
facies patterns and types change without seaward progradation of en vironments,
Deposits
most commonly encountered effect of the transgressive processes is erosion of the sediment-starved beach foreshore and shoreface deposits by wave action, leaving behind a scour surface capped by a thin lag of coarse intraclasts (beachrock clasts) or fossil debris. This lag may in turn be overlain by a thin burrowed or laminated lime grainstone/packstone package which represents deposition in shallow offshore marine or washover fan environments, respec-
233
tively. If the transgression takes place regionally along the strandline, and landward marine inundation is exten sive, a broad, regionally correlative crossbedded sand will develop . This sand would normally fine upwards in to other low energy marine units. Although it would pinch out in the zone of maximum updip transgres sion, its thickness would vary depend ing on the topography upon which it was deposited, and the relative rates of local transgression . Laporte ( 1 969)
studied a Lower Devonian (New York) transgressive carbonate se quence in which the upper Coeymans Formation (bioclastic lime grainstones and packstones) probably represents reworked strandline deposits which separated lagoonal (Manlius Forma tion) from deeper offshore marine (Kalkberg Formation) deposits. In essence, preservability potential of beach complex facies increases with depth below mean low water. Essentially all facies deposited above
234
Beach
F igure 1 ia-lsfand development on top oi elor�gate oolil"=l sh oal, Bahamas.
Figure 1 7b-8eac:h foreshore {tl} cap· pi ng tidal inlet (t) and ooUt� shoal (s) ae· (1 uence. Overlying units, subaerial crusi (dar1< layer), and supratidal dc:.lomlte (d), Indicate shoal was SIJba&rially �.xposed a.."ld formed an island.
Beach
235
Figure 1 7c-Eiongate reef forming core of emergent beach·dominated island. Only a thin veneer of ca rbonate sand and rubble separate the beach foreshore from underlying consol idated reef rock; seaward facing side is toward bottom of photo.
effective wave base (beach foreshore and shoreface) are removed during transgression and marine inundation . The most preservable beach deposi tional facies, in descending order , are : (I) tidal inlet sequences , in cluding both tidal channel and all but the uppermost units of flood-tidal delta deposits. The latter would occur as elongate or semi-circular lenticular sand bodies beneath a regionally tran sgressive sand sheet , if present; (2) lowermost parts of ebb tidal deltas ; and (3) washover fans into the lagoon and backshore zone (these stand the best chance of being preserved if the beach is developed on a reefrock core, or behind a rigidly cemented beach). Small beaches that fringe a lagoon on the mainland and on the backshore side of a barrier might also be preserved because low-energy lagoonal or supratidal deposits will probably cap these sequences before effective wave scour takes place. However, their preservation, as is the case with washover fans, depends on the rate of subsidence and the rate at which transgression takes place. Beachrock formation, early fresh water cementation, and stabilization of beach sediment through soil for mation all help preserve the beach foreshore and backshore units from erosion. They therefore stand a better chance of being preserved during marine transgression than do their siliciclastic counterparts.
Electric Log Response
The geophysical log response (SP-resistivity; gamma ray-neutron/density) for the general beach model , neglecting burial ef fects, is shown in Figure 2. The com bined geophysical log shape can be described as an upright cone, and is identical in shape to its quartzose clastic counterpart (Shelton, 1 973). However, because of the primary dif ference in mineral ogic makeup of the sediment , early diagenetic alterations in carbonate beaches commonly create dramatic changes in their porosity and permeability characteristics, and thus give rise to geophysical log patterns that are drastically different from those predicted from the simple beach depositional model. It is for this reason that the subsurface geologist will, in most cases, have to initially describe a number of cores or well cuttings for the purpose of correlating log responses to specific lithologies or diagenetic modifications. If a spec trum of logs is available (sonic, neutron, density, etc .), then lithologic types , variations, and associations in specific beach settings can be iden ti fied and used to predict the trend and extent of beach associated porosi ty, and potential zones of stratigraphic and/or diagenetic trapping.
Figure 1 7d-Goral rubble beach foreshore on i s la nd simil a r to Figure 1 7b. Rubble consists of coral and fragments of cemented reef rock which accumulated during high-energy storm
activitv.
THE GENERAL BEACH ENVIRONMENT AS A POTENTIAL RESERVOIR
Single regressive beach sequences generally average 6 to 1 0 m thick, with sands comprising the upper 2 to 5 m (Shelton, 1 973). Their length can usually be measured in kilometers parallel to depositional strike with the width of the beach measured from hundreds of meters to kilometers in the case of a long progradational history. Significant vertical carbonate beach reservoir development can be achieved through the depositional stacking of beach sequences . Stacking is commonly accomplished at a hinge
236
Beach
Figure 1 7e- Low-energy beach on lagoonal side of island; note small waves and very shallow offshore environment; backshore bedding features here would be destroyed by intense rooti ng.
line, with localized growth faulting, or with continuous salt structural movement in combination with con tinuous regional subsidence and ap propriate sediment availability. In the case of beach sequence stacking, the potential reservoir will not be con tinuous, but will be broken into a series of isolated , stacked reservoirs separated by the low energy offshore and backshore facies of each beach package, as illustrated diagram matically in Figure 1 8 . The prime reservoir rock types are obviously the well-sorted grainstones of the foreshore and shoreface. As will be seen in the specific examples to follow, however, all beach · associated lithologies are potential reservoir rocks because of many commonly oc curring later diagenetic events .
Cretaceous Isolated Island Beach - Texas
Our first specific example is an isolated island beach associated with a Cretaceous rudist reef. This exam ple was chosen because of the ex cellent quarry exposures and classic development of beach structures and facies. The exposure is located atop Round Mountain northwest of the
village of Comanche, Comanche County, Texas (Fig. 1 9). The beach sequence is developed in the Lower Cretaceous Edwards Formation . It is associated with an isolated rudist patch reef located on a shallow marine shelf on the eastern flank of the Concho Arch (see Figs . 1 9 , 20). The beach itself is approximately 1 km long and built into normal marine shelf waters about 5 m deep (Fig . 20) . Three-dimensional quarry ex posures allow a view of beach geomorphic landforms not commonly seen in the ancient record, such as the large scale beach accretion units (Fig. 2 1 ) which mark the progradational episodes of the foreshore. A berm position is indicated by subtle dip toward the backshore at the top of some accretion units (Fig . 22) . The development of a thicker supratidal fill in the beach swales between major accretion bedsets is also displayed in Figure 22. A thin recessive clayey and chalky caliche dedolomite unit (Fig. 23) is exposed near the top of the quarry. This caliche dedolomite represents a Cretaceous soil profile (Moore et a!, 1 972). Above this soil horizon, and cap ping the sequence, is finely crystalline, thinly laminated dolomite which is 3 to 7 ft thick and exhibits
mud cracks, rip-up clasts (Fig. 24) , and bird's-eye fabrics. This dolomite is thought to represent the supratidal backshore depositional environment which prograded over the soil horizon shortly after it developed across the upper surface of the beach accretion units . The lithofacies developed in the sequence are shown diagrammatically in Figure 25, and can be recognized in the quarry wall photographs (Figs. 2 1 , 22, 23) . Figure 25 illustrates the accretion-bedded mollusk lime grainstones below the soil profile and dolomite. Accretion beds dip to the right (seaward) from 8 to 1 5 ° . The grainstones are coarse, laminated, and may be graded within individual accretion units (Fig . 26). Large col lapse structures and disc-shaped, reworked cobbles (Fig. 27) represent Cretaceous beachrock collapsed and torn up by storms and reincorporated into the sediments . In thin section, these cobbles commonly exhibit originally aragonite microsta1actitic beachrock cements (Fig. 28). The lime grainstones of the upper foreshore give way downward into more muddy rocks including mollusk lime packstones and wackestones (Fig. 29). The lower part of the sequence, which represents the marine offshore environment , is marked by small scale
Beach
c=J
Wackestone (Offshore
Mudstone--Wockestone Re>tric1ed Mudflat
logoono l to Supratidal
237
Marine)
II Coarse--Fine Groirtstone-- Packs.1011e L__j IJead, Fnr��ore- Short'foCI"' Fig u re 1 8-Diagram displaying the three-dimensional distribution of barri e r island facies and their relationships
to
surrounding units In a vertically stacked sequence.
festoon crossbedding, and numerous subhorizontal feeding burrows (Figs. 30, 3 1 ). The beach associated deposi tional environments represented by the rocks of this quarry are .sum marized in Figure 32. The porosity present in this beach at the time of deposition must have had a distribution similar to ;that il lustrated in Figure 32, with relatively low effective porosity in the fine grained supratidal muds, maximum effective porosity developed in the upper foreshore, and a progressive loss of this porosity downward into the offshore marine facies. A similar porosity distribution has been documented in Holocene quartzose clastic beach sequences. The actual measured porosity in this sequence (Fig. 32) is strikingly different from its original inferred distributional pat-
tern and illustrates dramatically the importance of diagenesis in porosity development in carbonate rock se quences . Dolomitization of the supratidal sequence and subsequent removal of remnant calcite resulted in the development of high porosities (up to 30%) and moderate permeabilities (30 to 40 md) (Fig. 33). The upper foreshore , the site of max imum effective porosity at the time of deposition, has undergone extensive early diagenesis, including beachrock cementation and early silicification associated with reflux of waters from the overlying supratidal en vironments . The result, as shown in Figure 34, is a tightly cemented silicified lime grainstone. The grainstones, packstones, and pelletal wackestones below the intertidal zone were relatively unaffected by very ear ly diagenesis and hence retained max imum primary effective porosity into early burial . Lower Cretaceous un conformities along the margins of the East Texas Basin exposed the se-
Fig u re 1 9-Map showing Round Moun tain location and main Early Cretaceous structural elements.
238
Beach
Local c
. 0') 0 rJ) .G>
i::
0 .:0
<(
"' :I
0 Q) u 0 Q) ...
u
,_ Cll
� 0
.....
. 0
· ;;; ">
6
0')
... :I ..0 "' ..:.t. u
· ;:;
Qj
11... l.L.
Fig u re 20- D iagrammat ic s k etc h of beach setting at Round Mountain .
Fig u re 21 -Exposure showing laminated seaward dipping ( 1 5°) beach foreshore accretio n units. Note tabular fore s e ts between dashed l i nes near center of photo.
Figure 22-Round Mountain quwry wall showing foreshore accretion beds (f), bac k s h o re d ipping berm sequences (b), and thicker supratidal dolomites (s) in backshore topog raph ic low.
Beach
239
Figure 24-Laminated a nd mudcracked supratidal dolomite facies disp laying disrupted and t r u nc ate d laminations near its contact with the underlying caliche dedolomite.
Present Land Surface
Figure 23-Beach backshore ca l iche dedolomite (c) betwee n underlying beach foreshore unit an d overly i n g s u p ra tid a l do l omite .
Figure 25-Distribution of lithofacies types in the Round M ou ntai n beach se quence, Com anche, Texas.
� �
• quence to fresh meteoric water before inversion of the aragonitic bioclastic grains of the lower foreshore to calcite, resulting in the development of extensive fabric-selective moldic porosity (Fig. 35). Molds were selec tively developed in aragonitic caprinid (rudistid) grains. Calcite cement , originating from the dissolution of adjacent aragonitic grains, occluded primary intergranular porosity. Preserved porosity in the sequence is almost ent irel y secondary in origin, and all primary porosity has been destroyed. Figures 36 and 37 diagrammatically illustrate the diagenetic history and resulting porosity evolution of this sequence (published originally by Moore et al, 1 97 1 ) .
Laminated Dolomite Caliche-Clay Dedolomite
� � Lime Grainstone with
1!111
Laminations and Accretion Bedding
Rudist Lime Packstone with Accretion and Festoon Cross-Bedding Pellet Lime Packstone with Burrows and Festoon Cross-Bedding
We will compare the above exam ple with other ancient carbonate
Cretaceous Mainland Beach Central Texas
beach sequences, pointing out their
The classic example of a mainland beach sequence is developed in the outcrop belt of the Cow Creek Limestone (Fig. 38). This unit was deposited during Middle Trinity (Ear ly Cretaceous) time around the margin of the Llano Uplift in central Texas (Stricklin and Smith, 1 972; In-
contrasting depositional settings, sedi ment types, and diagenetic modifica tions, especially those which have modified the original primary porosi ty distribution.
-
240
Beach
Figure 27-Cobbles and boulde r-sized beachrock clasts at base of foreshore r ubble z one . Figure 26-Eve n ly lami nated mollusk lime grainstone beds of beach foreshore sequence.
den, 1 972, 1 974). The Middle Trinity is a carbonate-clastic package which represents deposits formed during one transgressive-regressive cycle (Fig. 39) on the broad Lower Cretaceous shelf that extended eastward toward the Gulf of Mexico. Carbonate strata in each Trinity cycle represent the marginal marine and offshore equivalents of alluvial-deltaic units deposited closer to the source areas (Figs. 39, 40) . The Cow Creek Limestone and Hammett Shale represent a high energy beach and its equivalent lower energy offshore deposits, respectively. The beach extends as a continuous regressive sheet o f carbonate sand from its pinchout near Marble Falls, Texas, to over 5 m thick at its seaward-most (easternmost) extent in outcrop (Fig. 41); Loucks ( 1 977) has published on its equivalent in the sub surface to the south-southeast of this area. Rigorous waves and strong longshore currents are indicated by coarse-grained, steeply dipping (8 to 12°), evenly laminated lime grainstone beds (Fig. 42a, b, c) and underlying festoon crossbedded lime packstone-grainstone units (Fig . 43a,
b). Abundant siliciclastics in these deposits and finer grained quartz sands in the shallow offshore lime packstone equivalents (Figs. 40, 44a), indicate significant detrital influx from the nearby source area (Llano Uplift) . Oysters and other mollusks constitute most of the bioclastic material in all facies, suggesting that the offshore environments were not conducive to either ooid formation or reef growth . A regionally extensive thin oyster biostrome, at the contact between the Cow Creek Limestone and Hammett Shale, and an overlying lime/packstone shoal deposit contain ing abundant disarticulated Trigonia valves and other large mollusks (Figs. 40, 44b) , were the probable sources of most of the beach bioclastic debris . The Hammett Shale consists primarily of muddy lime and dolomite wackestone/packstone (Figs. 40, 44c) and represents units deposited in deeper offshore en vironments which were below effec tive wavebase The backshore facies of this beach consists of sporadically developed washover deposits (lime grainstones, Fig. 45a), nodular caliche paleosols .
Fig u re 28- M ic rostalactitic beach rock cement (ms) t h ou g h t to be or i g i n al ly aragonite (scale equa l s . 1 25 m m) .
(Figs . l 3e, 45b) and very thin, disrupted , supratidal marsh and pond deposits (dolomitic Chara lime wackestones , Fig . 45 b) . These grade vertically and l aterally landward into alluvial-plain red mudstones and arkosic sandstone channel-fill deposits (Figs. 40, 4 1 ) . The lack of a well developed supratidal marsh facies probably reflects a paucity of car-
Beach
241
Figure 29-Fine g r a i n e d sorted fossil fragment-pellet lime packstone deposited in the offshore environment (scale ,
equals . 1 25 mm).
Figu re 30-Small·scale festoon crossbeds developed in sub·beach u n its; these ind icate that the offs h o r e environ· ment was periodically swept by wave· induced currents.
bonate mud in the offshore environ ment due to extremely high-energy conditions or lack of production.
primary porosity in the foreshore and shoreface lime grainstones. (2) Soil-forming processes resulted in the upper part of the foreshore
Diagenesis
units being cemented with , and
The following is an outline of the diagenetic modifications that were im posed on the beach sediment prior to any significant burial . ( 1) Beachrock cementation occlud ed only a minor amount of original
replaced by, micrite and dolomicrite, and capped by a nodular caliche (Fig . 45b) . (3) The early development of a stable fresh ground water lens displaced sea water in the sediments
to a depth of 10 to 40 ft and brought about the following modifications: (a) abundant secondary bimoldic porosity in the foreshore, decreasing downward into the shoreface se quence; (b) a downward increase, into shoreface and offshore facies, of recrystallization/inversion fabrics in grains that were originally aragonite; (c) a downward decrease in the fre quency and thickness of bladed ce-
242
Beach
Figure 31 a-Festoon
cross-bedded pellet grainstones to packston es in abr upt contact with burrowed lime packstones representing offshore shelf facies in front of Round Mountain beach sequence, Lower Cretaceous, Edwards Formation . lime
Figure 31 b-Branching burrows in lime packstones and wackestones below foreshore i ndicate deposition in low energy subtidal environment.
. -Ill bol��i"te ��up Clasts and Lciin�ti�{\ -<:: i ;\?;... . . • =��deij �ilu�clime GroinUoneS .,.;iA��--Bc·.�:?Jill,!)"_ •:iJlii " "L��()fiO<)S :::
·
·.
..
k�¥1
..
.�odist.: Pellet Lime Grainstone
. Pell� ;· Lime
w/.A�ftO!l.B�ioodli�s�� t:i':i:!SS:i:JWd flj} . ·
Grr;sinston�ackst� wl��i� ·
Figure 32-Diagram illustrating deposi and resultant lithofacies exposed in Round Mountain ...;.:;.;, ....--' .,_ ..;.;.;. -.;..; � ""'"""= � '"'""'" � = ,.;.;.;. � � = quarry, Comanche, Texas. ·:<
··:·
t io na l environments
<---'-'--'....:. c.:... ....c _ ... _ _ --..: _ _ _ _
ment grain coatings ; and (d) plugging of some secondary porosity and small primary pores with iron-free equant calcite cements (Fig. 43b) . All of these diagenetic alterations and gradients are the result of under saturated (with respect to aragonite and calcite) rain water becoming pro-
gressively more saturated with respect to calcite as it percolated downward through the sequence, and the mixing of this water with sea water near the fringes of the water lens, thereby in creasing the magnesium content of the water and inhibiting the precipita tion of calcite cements. During progradation of the alluvial system over the beach, a regional
ground water flow system replaced the p re-exi st i ng local one and caused: Figure 33-Laminated supratidal ( l ) dolomitization of the underlying dolomite showing only rare moldic Hammett Shale o ffshore sequence of porosity (wh ite ci rcles}; most porosity clayed lime wackestones and (300fo} is microintercrystalline, however, mudstones, thus creating abundant and is not visible in this photograph secondary microintercrystalline (scale equals .065 mm).
Beach
243
Figure 34-Partially silicified l ime grainstone; silica cement fills primary and moldic voids after i n itial cementa t ion with coa rs e bladed calcite cement, of presumably fresh water origin (scale equals . 1 25 mm).
Figure 35-Extens ive moldic porosity in the lowermost portion of the beach foreshore sequ en c e ; porosity is developed in leached rudistid (m o llu s k) grains. Note the lack of any remaining primary porosity between the molds (scale equals 25 mm).
Brines
Fresh Meteoric and Morine
Figure 36-Diagenetic processes and products resu lting from the syn diagenetic modification of the Round Mountain beach sequence. Note especially the modification to po rosity distribution .
porosity between the dolomite rhombs; and (2) precipitation of iron free equant cement (in the foreshore) and iron-rich equant cement (lower foreshore and underlying units) oc cluding all remaining biomoldic and primary intergranular porosities. Mississippian of Kentucky
Mississippian carbonates (Newman Formation) were deposited in a bar-
rier complex of beaches and tidal bar belt deposits near the axis of the Waverly Arch in eastern Kentucky (Figs . 46, 47a, b) (Perm et a! , 1 97 1 ) . They separate pelletal lime mudstones and wackestones of lagoonal origin to the south from open marine, thinly bedded bioclastic carbonates and red and green shales to the north (Fig. 47b). This barrier complex illustrates the effect that local structural and erosional t opograph y has on both beach development (Fig. 47b), and the early diagenesis within beaches and surrounding deposits (Canfield , 1 974). Studies o n similar Mississip-
pian shoal deposits have been carried out in Indiana by Carr ( 1 973) and in the Bridgeport oil field of eastern Il linois by Choquette and Steinen ( 1 980). Most of the barrier complex is made up of lime grainstones contain ing abundant , well-rounded and sorted intraclasts, oolites, bryozoans and mollusks (Fig. 48a, b) . Individual depositional packages of these are up to 20 ft thick and 1 km long, but their width is unknown because of the lack of lateral outcrop control . They contain medium- to large-scale fes toon crossbeds in their central and
244
Beach
Figure 39-Stratigraphic relationships between Trinity age rocks on east flank of Llano Upl ift; units on the right are carbonate-rich and were deposited farther away from the source areas than those on the left (after Lozo and Stricklin,
1 956).
Figure 38-Diagram showing the Cow Creek Limestone study the eastern flank of the Llano Uplift, just nort h of the San Ma rcos Arch.
a rea on
basal portions . These crossbeds are
posed of poorly sorted fossil frag
oriented perpendicular to , and grade
ment lime packstone (Fig. 48e).
upward and laterally into large scale, low angle (less than 10°) accret ion beds (Figs. 47d, 48c) that are com monly laminated, or graded, and con tain occasional keystone vugs (Fig . 7d) . The basal and distal ends of these accretion beds are usually com-
Beaches and tidal channels developed on the immediate flanks of the topographic high, whereas tidal bar belt units formed in a topographic low to the east and event ually built up into islands defined by the beach facies and underlying units (Fig. 48d,
f).
Beaches throughout this complex
are capped by one or a combination of: ( 1 ) convex-up spillover crossbeds representing storm washovers; (2) low energy supratidal carbonates; (3) green clayey insoluble residue seams, and laminated subaerial calcrete crusts (Figs. 1 3g, 49) containing
Beach
245
Figure 40-General depositional model for H en se l Sandstone-Cow Cree k Limestone-Hammett S ha l e . This deposi· tion pattern generally holds true for all the Trinity sequences deposited near the Llano Upl ift.
coated grains, teepee structures (Fig. 50) and a variety of vadose features (Fig. 14d) ; and (4) low energy open marine (red and green shales; lime packstone) or lagoonal (lime wackestone) facies (Fig. 48d). The fact that low energy subtidal facies directly overlie these beaches without a basal lag indicates that beaches can be preserved during transgression if they form in a lagoonal setting or are protected in some manner (for exam ple by spit accretion, early cementa tion) from erosion by direct wave approach. Diagenesis
The diagenetic history of the se quence is outlined as follows. ( 1 ) Beachrock cementation occlud ed minor intra- and intergranular porosities in the beach and shoal units (Fig. l 2b). (2) Intensive weathering took place in the upper shoal and beach deposits
before burial (Fig. 49), resulting in : (a) formation of a thick brecciated, and partially silicified , laminated subaerial crust containing vadose (stalactitic, meniscus) cements, root tubules, and numerous other car bonate soil features (Fig . 1 3g) ; and (b) development of narrow bedding plane (desiccation?) fractures and small subvertical solution channels ex tending downward through the shoal and into the offshore bioclastic lime packstone facies (Fig . 1 3h). (3) During the weathering period, a fresh water lens occupied the shoal and small portions of the underlying and surrounding facies , and caused (Fig. 49) : (a) the precipitation of abundant iron-free bladed cements in only the upper part of the shoal (Fig. 48a); and (b) slightly later precipita tion of iron-free equant cement which plugged much of the remaining primary porosity (Fig. 48a). Little if any secondary porosity was
created during this diagenetic episode except for the solution channels and fractures mentioned above. During shallow burial, the lower part of the shoal and underlying packstone facies underwent compac tion (tight packing fabric) and pressure solution (abundant grain in terpenetrations and microstylolites ; Fig. 48b) in response to the over burden pressure being exerted by the weight of the overlying prograding facies. Compaction effects were minimal in the upper shoal and beach grainstones because of cementation and formation of a rigid framework during earlier diagenesis. Cementation by iron-rich equant cements occluded all remaining porosity at a slightly later time (Fig. 48b). Cretaceous Shelf Margin - Texas
The subsurface Stuart City Trend of the Texas Gulf Coast is a facies association that represents the
246
Beach
Figure 4 1 -Stratig raphic c ross-section of M iddle Trinity rock sequence along eastern margin of Llano Uplift.
Fig u re 42a-8equence of beach foreshore accretion beds dipping toward background of photo and indi cating pro· g radati on to the east.
Fig u re 42b-Evenly lami nated, well sorted, coarse- and medium-grained mollusk l i me grainstone.
Aptian-Albian/Lower Cretaceous shelf margin (Figs. 19, 5 1 ) . On the shelfward side of this complex (Bebout and Loucks, 1 974), beaches are abundant in the uppermost part of the Edwards Formation (Figs. 5 1 , 52) . They occur in stacked, thin, nar row regressive or transgressive se quences parallel to depositional strike. These potential reservoir packages attain a thickness of 75 m and have a length of 1 0 to 1 5 km, and width of up to 4 km. Their distribution is largely controlled by the position of older structural highs and organic reef mounds, both of
which provided the minor but signifi cant topographic relief to the sea floor necessary for the intense win nowing action of waves and currents, and the concentration and buildup of carbonate sands in these areas . The resultant sand bodies form significant gas reservoirs throughout a large part of the trend. The best pro duction from these units occurs in beach foreshore and washover lime grainstones, in a relatively narrow zone paralleling depositional strike. These grainstones pinch out into tight finer-grained supratidal and lagoonal units in a shelfward direction .
Beach
247
Figure 42c-Well sorted and rounded
medium g rained lime grainstone (scale equa l s .25 mm).
Fig u re 43a-Medium-large scale festoon crossbeds o riented perpendicular to trend of beach foreshore units.
Toward the shelf edge they grade into offshore lime wackestones and packstones carrying a diverse marine fauna.
In cores , porous b each foreshore .3 to 3 m thick, are typically tan, and have even horizontal lamina ti o ns (Fig. 53b, c) or are homogeneous. They are composed o f well-sorted or bimodal fossil fragment-intraclast-algal coated grain lime grainstones (Fig . 53b, c) , many of which display large fenestral-like, deposits
cement-filled , keystone vugs (Fig. 53a). In regressive sequences, these grainstones are overlain by thin supratidal algal-laminated lime mudstones and fractured soil horizons (Fig. 54a) . They fine downward into
offshore, bioturbated , algai-Toucasid and Chondradontid lime packstones (Fig . 54b , c) or lagoonal fine-grained, wispy bedded, calcisphere-algal-pellet lime wackstones (Fig. 54d, e) . The lack of a crossbedded shoreface se quence indicates that longshore cur-
Figure 43b-Medium·coarse. poorly sorted mollusk lime grainstone contain· ing abundant quartz sand (scale equals .5 mm). ·
248
Beach
F igur� 44a-Very a renaceoua, birnodaily sorted, mollusk lime pack!"ltone fmm s.h�llow dlshOre rnariu� e rwironm�nt; bimo<Jality ia pro�o� due to tranaport or t ine qua•tz sa.nd hto an a tl"..a whe re coarse mol l usk frag mE;;Yo ts were being prod,Jc�; Middl� c'J;m Creek Limestor1e (scare equalS .5 mm). Figure 4-lb-Upper surface of shoal ume packstone deposit di:;piayir.g abundant large TtigorJI:J values; Lower Cow Creek Limestone.
rents were either nonexistent or so weak that deposits laid down by them were homogenized by later bioturba tion . Tidal inlet fills (.3 to 7 m thick)
are somewhat coarser overall, and Figure 44c-81otvrbate<:l, p<:�orly sorr.oo fragment c lay$y dolomite wsckf}stone.paokstone; unit wae depositoo in ve ry low energy o;n. vi ronments; Hammett Share: leng th of slab f!Que�ls 10 em. oyste;
contain abundant bimodal, oncolite
supratidal, or shallow restricted lagoonal deposits.
Transgressive beach sequences are made up of multiple storm washovers that overlie scoured supratidal deposits. Individual washovers are
lime grainstones (Fig. 55a, b) . They
thin (.3 to 1 m thick) and are
typically become finer grained up
represented by a lowermost thin,
ward from a sharp basal scour con
fossil-intraclast lime packstone lag and an overlying finer grained and better sorted foreshore-type lime
tact with a wide variety of lithologies,
and are overlain by beach foreshore,
Beach
249
Figure 45a-Top of beach accretion u n its (dipping toward right) is partially truncated by c hanne l deposit (left center) which in turn is overlain by land wa rd (left) d i ppi ng washover u n it (arrow).
Figure 45b-Nodular caliche paleosol, composed of diagenetically formed lime wackestone, ove rlain by distorted dolomite mudstone of sup ratidal marsh a nd pond origin.
grainstone. These units are capped by either supratidal or lagoonal (tan calcisphere and Chondradonta bear ing black lime wackestones) sediments. Transgressive beach washovers cannot be differentiated from associated small tidal channel deposits in individual cores because the sediments and sequence for both are essentially the same. Although these beaches occur in close association with rudist reefs, they contain only a limited on-reef fauna, thus suggesting that they formed in a somewhat restricted backreef environment. This might be the result of transport barriers, such as deeper water channels, across which sediment could not be moved shelfward from the reefs.
Diagenesis
The generalized early to late diagenetic modification of this se quence is outlined below. ( 1 ) Mild weathering episodes created minor fracture porosity in rocks deposited in the supratidal zones of the stacked barriers (Fig . 54a). (2) Fresh water lenses developed in these barrier beach complexes causing the leaching of aragonitic mollusk fragments and minor early cementa tion by bladed calcite cement . Most moldic porosity was eventually filled during this stage with bladed and equant cements (Fig. 53c) . (3) Shortly after deposition (Cenomanian time) and during shallow burial, sea level fell
Figure 46-Location map of Newman
Limestone barrier sequences in eastern Kentucky.
250
Beach
·-.
4.7�-G�neral deposi1ioMI J"rl()(jel tor offsr.ore, barri�r, and oack t.arriP.r ca rbonate seqvanc�s {tmm f:.;�rm At a.l,
FiglJr�
r� $�.ndt�Otl lll. or � t •OOv4.rl�iH¢ 1--:--
· ·
·1 :.� ho.l e
<�Orh;
COll bOr�IO.O, oo�r'f>"!- .." r 3 • 1'1 -l d (uc l ·tOG �o'IOI CJt::a fr-s,)rr.l!n:�. .:.:NbLJtte!e cl'l!ft:s. c.: he rt r:ln�l� 11n l;1 � " '4r. 2 . Qt'AJ rtS.} ..:,. :u � n:u: � . flrQ- nr';.)!f'.:.l! :rn•cr�Q :.at'lu r.A u � · ��
T i •S i.l l - fl�t <..3'�";:;.lc!:
1 97 1 }.
����� MOol1 ..)
e �Jbne. : t'i eJ..p o�•�te
- ······
. .. ---
Quartz r;c,;n Oolite gri:l.in:r.tone P a c li st O l1 !!
D
!o
m u dstone
S o.� pr ati�al d olo'l'lite
E3 � � B
E: x p o s 1.1 r e
{o::: up'n'! !d-.'21
ZD"•t'
..... ·······-· .
(�!Jb�rtnJ cn16tS
� 3 1 1 Cn.;a)
. . . - ·----------· -,
c r 1.1 sta
Storm dcpo:;i1S Cross -b�d d i n g
r � 9Ptc<� !' t r v cture!;
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ � _ -_ ._ � � � � c __ . '_ 'c _ " _ e _ _ �u _ r, _ a _ � _ _ .�� - -- - · - - - · · -- - ---
Figure 47b-Oross-e%tlott showing local iacies varia!ior.s in c.-lrboMte$ 3round erosional topographic hi gh on Borden Siltstone.
�o.io:O"=" lc .,,,d
��
I
�
--�---�--� 1�
Beach
251
Figure 48a-Well sorted ooiite·intraclast lime g rainstone from barrier beach ; ea rly diagen�tic precipitation of iron-free (pink) bladed and equant spar occluded most porosity; later precipitation of i ron-rich {pu rple) equant calcite spar plugged re· main ing porosity (sca le equals .25 mm).
Figure 48b-Sorted, oolite-fossil frag· ment lime g rainstone f rom lower part of barrier sequt�nca; because of the lack of precipitation of early i ron-free cement a rigid framework was not c reated in the sediment; grain interpenetrations and close packing fabric resu lted from com· paction : late iron-rich (purple) equant calcite occluded remaining pore space (scale equals .25 mm).
significantly, resulting in the nearly total exposure of the Lower Cretaceous shelf. Fresh water displac ed the previous pore waters in the rocks beneath this area and gave rise to a massive dissolution episode in the shelf margin zone. In the beach and tidal channel lime grainstone packstone facies, earlier cements were partially dissolved and secondary porosity was created in the form of vugs (Fig. 55b), and more important-
ly, as microintercrystalline porosity in the algal coatings of individual grains. Solution channels and larger cavernous voids formed in the fine grained lagoonal and offshore marine facies surrounding the beaches . (4) During deeper burial diagenesis, very coarse equant spar was precipitated in the large secondary voids, leaving only microinter crystalline porosity and minor small vugs as the effective porosity in the
beach sequence. Oil inclusions are present in the equant spar suggesting that hydrocarbon m igration into the sequence occurred at approximately the same time as final cementation . Mission Canyon Formation Williston Basin
The Miss ion Canyon Formation is one of a series of regressive cycles that was deposited in the Williston Basin during Mississippian time (Fig.
252
Beach
Figure 48c-Low angle accretion beds of tidal bar and beach sequence. Dark unit in center of out c rop is lime packstone partially altered by weather· ing into subaeria! crust.
Figure 48d-lower half of diagram represents depositional characteristics noted in outcrop photos 48c and 48f.
OUTCROP 10
O o l o rn l l e
1-e-j
Caliche
Fen estral f a b r i c s
E23 Wavy
beds
� Scour surface
f:m Teepees l2:2l Low
a n d nodu l e s
angl e accreti o n
� Fes-1ooM
cr-o�s
beds
O rthoquartz ite
beds
Calcisphere pellet pack$10ne
Well s orted Oo l it i c g r a i nst o n e Poo r l y �< o r t e d
bioclastic
p ac k sto n e
..._ __ � ---
- ··-· · .. ·· - -
(Fig . 56b, c) . The cycles were formed
of these cycles (Fig. 56c). It produces from many fields along the periphery of the northern half of the basin, and significant fields are presently being discovered in the southwestern part of the basin. Wittstrom and Hagemeier ( 1 978) and Lindsay and Kendall ( 1 980) have studied in deta il the Mis
in an environmental setting very
sion Canyon sequence at Little Knife
lagoonal carbonates are found im
similar to the Persian Gulf Coast
Fie l d (Fig. 56a) and Gerhard and
mediately beneath the Bottineau
(Purser and Evans, 1 973) and
others ( 1 978) have reported on the
anhydrite (Fig . 56c). Barriers actually
presumably broad sabkhas extended outward from the basin m argins . The Mission Canyon is one of the
Glenburn Field in Bottineau County. Pinchouts of porous facies into the Bottineau evaporite (anhydrite), com
most prolific hydrocarbon producers
bined with anhydrite plugging, are the
ranged from beaches to medium and low energy tidal flats depending on the amount of wave and tidal current energy the strandline was subjected
56a, b). Each cycle has a somewhat unique set of subfacies and porosity that characterize it, but along the Mississippian basin all margins con tain a barrier system, a carbonate mud and evaporite filled lagoon, and low energy offshore carbonate muds
most common reservoir seals in the Mission Canyon in both areas. The following discussion is a summary of the lithologic and reservoir characteristics that typi fy the se quence in the southwestern portion o f
the basin. Either barrier sequences or
Beach
253
Figure 48e-Fi ne-medium g rained, poor ly sorted, pellet·fossil fragment lime packstone irom basal and distal P.nds of accretion units in 48c {scale equals .25
mm).
FigiJre 48f-Tidal bar belt (t) sequence grading upward in middle of photo i nto beach (b) unit which is partially altered into and capped by a subaerial crust (sc) and Stlpratldal dolomite {s).
to. Beaches are made up of buff, well-sorted and rounded pisolitic
irregular micritic laminations and radial-fibrous structure, indicating
Beaches and tidal flats grade downward on the basinward side into
oolitic-intraclastic lime grainstones
that they formed in a hypersaline low
a restricted marine, tan, bioturbated
(Fig. 57a, b, c) , whereas tidal flat
energy environment (Lareau and
and wispy laminated, mollusk-spicule
units are buff to brown, poorly
Purser, 1973); however, many may
calcisphere dolomite
sorted , lime packstones to
have formed as a result of weathering
wackestones with fenestrae and desic
and the formation of paleosols
cation fractures (Fig. 57d). Both at
(Gerhardt et al, 1 978) . Units contain
tain a maximum thickness of approx imately
6
m. The ooids and pisoids in
these deposits typically display highly
wacke stone/packstone (Fig. 58a, b) , which
ing these grains have previously been in the literature as algal
was deposited in a restricted marine environment. Below this facies are brown and black, laminated and bioturbated, siliceous bryozoan
1 966).
brachiopod-crinoid lime packstones
referred to
reefs (Hansen,
254
Beach
OUTCROP 10
f"'-"':!fJ
I3:ZJ �
L a m i n at e d s u baerial crust
(;;.t;'H C h erlifi e d
0 D o l o m it i c
s u baer i al c r ust
c a l i c he
Siji$ EZ]
I r o n - r i c h equ ant c "l l dle
�
Nodu l es
T e e p e e str u ct u r es S cour s urface
cements
l ro n f r e e b l a d e d a n d e q u ant calc it e cements
Figure 49-D iagenetic modifications to tidal bar belt and beach sequence. Posi tion of permanent ground water table can be approximated by contact bet ween iron-rich equant and i ron-free blad ed calcite cements.
Fig u re 50-Teepee structu res developed
in supratidal unit as a resu lt of calichification.
(Fig. 58c, d), which presumably represent the open marine· environ ment in the deeper part of the basin . In a landward direction , the barrier system grades into tan homogeneous and burrowed calcisphere-pellet dolomite mudstones and wackestones of restricted lagoonal origin (Fig. 59a , b, c) . Both the offshore and lagoonal carbonate facies are similar to those that developed in association with
Mississippian barriers in the Illinois Basin (Choquette and St ein en , 1 980). Thin supratidal dolomites and pisolitic laminated subaerial crusts (paleosols) cap, and are intermixed with, the uppermost lagoonal and barrier facies.
The lagoonal dolomites commonly contain abundant microintercrystal line and moldic porosity (Fig. 59a, c) and form the main reservoirs at Rough Rider, Little Knife, Fryburg and many other fields . The high energy beach sequence proper , which is normally not dolomitized, contains little, if any, predictable porosity.
occluded primary porosity in the bar rier beach oolite-pisolite lime grainstones and packstones . (2) Weathering took place mostly in the backshore and associated tidal delta areas, forming subaerial crusts with pisolites and other paleosol features. (3) In some areas (Bottineau Coun ty oil fields), partial dolomitization
Diagenesis - Mission Canyon
took place in the backbarrier
Formation, Little Knife
supratidal zone, thus creating ex cellent reservoirs with secondary porosity in paleosols, tidal delta, supratidal, and high subtidal facies. (4) Cementation in poorly developed fresh?-brackish? water lenses filled most primary pores in the
Area, and Glen burn
Field, Bottineau County A comparison of the diagenesis in two widely separated reservoir se quences follows below . ( I ) Minor beachrock cementation
Beach
255
(modified from Bebout & loucks) Reef Slope Lime Packstone -Wackestone (open marine fa�no)
STUART CITY R E E F TREN D B EACH RESERVOI R FACIES Upper Sruart City
Figure 5 1 - Depositional model for the Lower . Cretaceous shelf margin along the Texas ancestral Gulf of Mexico (after Bebout and Loucks, 1 974).
Lavaca Co. , Texas
Supratidal limestones.
D
Beach, Tidal Channel , and Washover Grainstones
lagoonal and Offshore
Padcston6$-Wack6$tones
Figure 52-Typical geophysical log penetrating the upper Stu art City stack ed beach sequences along shelf marg i n trend, Lavaca County, Texas.
256
Beach
Hgure 53b-Wel l-aortsd pe1let-mlllolld lime grafnc;too� from foreshore �::e· quenca; extoosive recrystall ization (especially mifiolids) due lo ear:y frer,h water diag�ne-sis (scale equals .25 mm).
5 :� a-Foreshote �cpmm of la mi nated tosSl l -peaet-algal c:Mti'!d gr.;J n l i me gm1nsrone. CMrser grained l�rniM· rlons t:lisplay ra rge, som�wn�t irregWlr xeysrcne vugs Mw filled with sparry ��Ieite cement; atluManr mlc ral:'ltP.r· t.rystalline poro�lty ln algal coatings on grains (s�a:e In Inches).
Figure
Figure 53c-t-liediunwery aoarsfl foee il lragmli:lnt-intrac l ast lima grainstone: gastropod has thick algal coatir.g; ahun· dant rnicrointercryste lline porosity in algal coatings and i n traclasts; lnose packing is probably due to e,.q rly diagenetic total grs.ir. disoolutlon, and partial collapse of loosely cemented sed iment (scale equals 1 .0 mm).
Beach
Figure 54a-Miliolid lime mudstone of supratidal origin displaying vertical frac . tures. sma l ! solution channels. and wispy, dark colo�ed lami natiCY.Is formed through sOil processes (s cale in inches).
Figure
54b- Lagoonal toucasid lime
packstone-wacKestone w i th finer grained matrix of poorly sorted pellets, algal coaled g rains and fossil fragments (scale in inches}.
grainstone-packstone facies in fields such as Little Knife; minor excellent (200Jo) primary porosity is occasional ly preserved (Fig. 57a) , but generally cannot be predi cted . In the Glenb urn Field the beach sequence contains ab undant primary porosity, thus making the beach lime grainstone facies one of the main reservoirs (Gerhard et al, 1 978). (5) During shallow burial beneath the Bottineau Anhydrite, dolomitiza tion gave rise to biomoldic porosity (in calcispheres, spicules, and mollusks) and microintercrystalline porosity in the lagoonal and restricted marine fossil wackestones and mudstones (Figs. 58, 59). In the Little Knife area, these facies make up the main reservoir and have up to 1 5 0Jo porosity and good permeability. Dolomitization probably took place through seepage refluxion, a process Figure &4d- Restricted' lagoonal 1an. in which lagoonal high-density, wispy-laminated, fossil f ragm ent-algal · magnesium-rich brines percolate OP.I Iflt l i mP. Wi'l �k.�tonP. (A('d'JI� ln inn h ��). seaward through porous sediments, replacing them with dolomite along the way. (6) During and shortly after dolomitization , anhydrite replaced grains and cements, and plugged
257
54c - ·Lagoonal chondradontid lime pac kstone·wackestone with line grained fossil fragment and pellet-rich matlix (scale in inches).
Figure
much of the remaining primary porosity as well as some of the sec ondary porosity created through dolomitization.
(7) Deeper burial diagenesis resulted in minor dissolution of anhydrite , formation of stylolites, and fracturing. CONCLUSIONS
Carbonate beach units are probably quite common in limestone sequences recording deposition on shallow marine platforms . They have only rarely b een recognized, and have more than likely been grouped together with other calcarenite units of diverse origins, even though they carry a distinctive suite of sedimen tary structures which readily allows them to be identified as beach
deposits. Carbonate sand deposits of beach origin may be locally developed around structural or topographic highs, patch reefs, or shoals, or may extend for miles essentially along the depositional strike of a basin or a fault block system . Other depositional
components of beach complexes (tidal deltas and channels, washover fans)
258
Beach
Fig ure 54e-Rest ri cted lagoon calcisphe re-foram-pe l let lime wackstone; m i l iolid foram and calc ispheres (upper right) have i r regular algal coatings (scale equals .25 mm).
Figu re 55a-Tidal channel deposit of bimoda l oncolite lime g ra i nstone (scale i n i nches ) .
Fig u re 55b-Bimodal, algal oncolite l i m e g rainstone with f i ne-medium g rained pellets and foss i l fragments (scale equals 1 .0 mm).
occur as lobes of carbonate sand which extend up depositional dip. In dividual beaches may attain a thickness of up to 1 0 to 1 5 m, depending on energy conditions and the depth of water they prograde into. In a regressive sequence, accretion beds, which are composed of laminated lime grainstone, mark the beach foreshore (wave swash) zone,
and are capped by any lithology car rying evidence of subaerial exposure (for example, paleosols , mudcracked supratidal dolomite). Commonly, the unit immediately beneath the laminated lime grainstones is a more poorly sorted lime grainstone to packstone which displays tabular and festoon crossbedding and represents deposition by longshore currents in the shoreface zone of the beach .
Whether or not this or other units are present in this stratigraphic position depends on the overall depositional setting (sea floor topography, wave and current energy flux) of the beach . In extremely low energy settings a crossbedded unit will not be present , and in reefal settings, beach foreshore units may directly overly a rubble zone which caps the lithified reef. In barrier island or high-energy shoal en-
Beach
259
MISSION CANYON
FACIES/POROSITY RELATIONSHIPS
WILUSTON BASIN
UTTLE KNIFE AREA
Amoco # l
Tachenko
Billings Co . , N. Oak.
100
200
a
lnde11 Mop of Willnton Benin lmocified cfter l'roctor arid Mocaull!f, 1968 and Wrtts.trotn and Hogeme�r• 1978, 1979)
"' w .. "' < ...
Figure 56-Mission Canyon formation location map and general stratigraphic relationships.
� Sorriel' and Neor Sorrier Corbooote• • lagoonal A.nhydrlhu . Soli Regional cross wction of MiniDI'I Canyon ocros.:s Nol'1h i)Qkote� portioi\S of the Williston Bosin. (Modified ofter Mddt·A�oni, J977)
vironments, a thick (5 to 20 m) crossbedded sequence, which represents tidal channel-tidal delta deposition , or the carbonate shoal unit itself, will underlie the beach foreshore facies. Evidence o f ex posure must be present in order to validate the interpretation that any of the above sequences represents a beach . Early cementation, by either marine, mixed, or fresh pore waters helps stabilize the beach sands and prevent their removal by erosion. Transgressive beach depositional
units are rarely preserved intact
because they are destroyed by wave
scour and erosion as the strandline position migrates landward. Instead,
the strandline facies is represented by a lag of fossil fragments or intraclasts
which occurs directly above a surface scoured into the underlying (typically
z UJ w �
"' "'
0 �
c
Figu re 57a-We ll-sorted, f i ne-medium grained oolite-intraclast lime gra instone. Pores are partially filled with early calcite cement, but abundant primary porosity remains (scale equals 1 .0 mm).
260
Beach
Fig lire S7b-$orted ool1te fragm9nt lime grainstme. Most ooids have fibroul$ structure arrsrtged radla,!y about th e i r nuclei, a factor whicn co ntributed to their tragment<\ry nature (sc ale SQtJ als .5 rmn}.
Figure 57r.:-Bimodal pellet-<:<>fite ·pioolite lim9 grainstone. lrregulur fibrous.
l� m ina.e of pisoids indicates oeposition in a hypersal in� eriVironmMt (scale equals 1 .0 mm).
Beach
261
Figure 57d-Poorly sorted intraclast-pellet-pisolite lime packstone· wackestone of low energy tidal flat se quence (scale equals 1 .o mm).
more landward deposited) facies. This lag is overlain by a thin laminated or bioturbated carbonate sand which fines upward into an offshore facies. In the end, the transgression is a fin ing upward, relatively continuous sheet of sand that extends over the area affected by the transgression . Except for those beaches developed on reefs, mudmounds, salt diapirs, or other structurally controlled topographic highs, most b each dominated sequences are parallel to the basin margins, and typically in crease in both frequency and gross thickness away from the center of the basin. Beaches may not be present continuously along depositional strike at any one time because of varying energy conditions and the availability of sand size (and coarser) carbonate in the offshore areas . If its prograda tional history across the shelf was in terrupted frequently , it will be represented by carbonate sand lenses. Oil and gas production from these lenses is often from stratigraphically formed traps because the beach units are overlain and grade laterally updip into nonpermeable evaporite, lime and dolomite mudstones, or shales. If a beach dominated strandline has had a long uninterrupted progradational history, it" will occur as· a relatively
continuous sheet sand, separating landward from more open marine facies . Cementation and other early diagenetic processes such as neomor phism and solution, give rise to dramatic changes in the distribution of porosity and alter the original permeability characteristics of the b each facies. In many cases, the units with the most original effective porosity (medium-coarse well sorted lime grainstones and packstones) are converted into rocks with good reser voir sealing potential whereas finer grained units (poorly sorted lime wackstones and mudstones) become the most desirable reservoirs, mainly as a result of dolomitization and solu tion . There is no set pattern of diagenetic history, porosity develop ment, or porosity destruction , in beach sequences. Each stratigraphic unit is unique within itself and must be treated as such. However, some of the primary factors governing the types and trends of diagenesis, and in part the resultant porosity distri bu tion of beach sequences are: ( 1 ) Original mineralogy - Many early diagenetic cements which precipitate out of fresh water form from calcium carbonate which is derived from the contemporaneous
Figure 58a-Tan , biotu rbated. pe l let, do lomite wac kestone with wi spy,' o rg anic-rich partings (sca le in i nches); rest ricted mari ne.
dissolution of aragonite contained within the sediment . Low- and high magnesium calcite are not as suscepti ble to dissolution as is aragonite in fresh water environments. Therefore, a beach sand rich in aragonitic ooids, coral, or algal grains, which is sub jected to early fresh water diagenesis, stands a greater chance of having its porosity and permeability character istics changed than does, for example, a beach sand composed of calcitic mollusk, crinoid, or bryozoan grains. (2) Beach setting - A regressive beach sequence will be bathed in widely varying water types during its early burial history, depending on whether it is attached to the
262
Beach
.·.
Figure 58b-Bioturbated fossi l fragment spicule dOlomite wackeS1one (scale equals .065 m m); restricted marine.
Figure SSe-Black and brown, laminated , b rac h i opod -c r i no i d lime packstone-wackestone (scale in inches); restricted marine.
mainland, backed by a lagoon, or developed on an atoll or offshore tec tonic structure. For example, beaches attached to a mainland might be sub jected to dissolution and cementation events that first take place in the local ground water systems that develop in the beach ridges, and secondly in the
Figure sed-Slightly sorted and biotur siliceous bryozoan-brachiopod minoid lime packstone from o ffshore open ma rine facies (scale equals . 5 mm).
bated
fresh ground water flow systems that occupy continental coastal regions. Beaches developed on atolls would be affected by the diagenesis taking place in the local fresh water lens, but would then be submerged into a marine pore water system during its early burial.
Figure 59a- Medium-coarsely crysta l l i ne calcisphere dolomite wackestone with abundant secondary and microi nter crystalline porosity (scale equals 1 .0 mm).
(3) Climate - In wet climates, abundant fresh water would be ex pected to pass through barrier and mainland beaches during their early burial histories. In very arid climates, evaporites would form in the backshore tidal flats, lagoons, or sabkhas on the landward side of the
Beach
263
Figure 59b-Calcisphere do l o mite wackestone with coarsely crysta l l i n e dolomite matrix (scale equals . 1 25 mm).
Figure 59c-Calcisphere dolomite wackestone with coarsely crystalline dolomite matrix; m o l d i c porosity developed out of leached calcispheres (sca l e equals .065 mm).
beach . Hypersaline waters derived from these environments would pass through the beach facies during their basinward flow, precipitate evaporites in primary and early secondary pores, as well as possibly dolomitize the finer grained near offshore facies. The resultant dolomites might form reservoirs or reservoir seals, depend ing on the degree of dolomitization, the size of the replacement dolomite crystals , and other factors. (4) Resident time in each diagenetic environment - The degree of altera tion the beach sequence suffers in each diagenetic environment (marine vadose and phreatic, mixed marine, local and regional fresh vadose and phreatic) is to a large extent depen dent on both the sedimentation and subsidence rates in the areas of beach formation. For example, in areas of extended beach progradation (sedimentation rate much greater than subsidence rate), the beach system would be subjected to the shallow burial diagenetic environments and pore fluid types for a much longer period of time than in a zone where beaches would be stacked along a h ingeline (sedimentation rate equals subsidence rate), in which case they would be subjected to the shallow diagenetic environments for a much shorter period of time, and would be more rapidly submerged into the
regional pore fluid systems occurring at depth.
ACKNOWLEDGMENTS The authors wish to express their ap preciation to John Horne and Peter Scholle for critically reviewing the manuscript and making numerous sugges tions which have led to its improvement . Thanks are also due to John Barwis, Mitch Harris, Miles Hayes, and numerous others for supplying the photographs which have enhanced the illustrative quali ty of this paper. S uperior Oil permitted the release of dat a on the Stuart City and Mission Canyon formations. Kathy Brown, formerly of MRO and Associates, graciously typed numerous versions of this manuscript.
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Petrology, v. 37, p. 556-591 . J. H., 1976, Internal geometry of Kiawah Island beach ridges, in M. 0. H ayes and T. W. Kana, eds. , Ter rigenous clastic depositional en vironments: AAPG Field Course, Univ. South Carolina Tech. Rept. No. 1 1-CRD, p. II/ 1 1 5-11/ 125. __ and J. H. Makurath , 1978, Recog nition of ancient tidal inlet sequences; an example from the Upper Silurian Keyser Limestone in Virginia: Sedimen tology, v. 25, p. 6 1 -82. Bebout, D. G . , and R. G. Loucks, 1 974, Stuart City Trend, Lower Cretaceous, south Texas, a carbonate shelf-margin model for hydrocarbon exploration: Austin, Tex., Bureau Econ. Geol. Rept. No. 78 , 80 p . Bernard, H. A., R. J . Leblanc, and C. F. Major, 1962, Recent and Pleistocene geology of southeast Texas, in Geology of the Gulf Coast and central Texas and guidebook of excursions: Houston, Tex., Houston Geol. Soc .-Geol. Soc. America Ann. Mtg . , p. 1 75-205. Boutte, 1 969, Callahan carbonate-sand c omplex , west-central Texas, in C. Moore, ed . , Depositional environments and depositional history, Lower
Barwis,
Cretaceous shallow shelf carbonate se
quence, west central Texas: Dallas, Tex., Dallas Geol. Soc., p. 40-74 . Campbell , C. V., 1 97 1 , Depositional
model-Upper Cretaceous Gallup beach shoreline, Ship Rock area, northwestern New Mexico : Jour. Sed. Petrology, v. 4 1 , p . 395-409. Canfield, J. C . , 1975, A depositional
264
Beach
model for the Middle and Lower Newman Formation near Olive Hill, Kentucky: Columbia, Univ. South Carolina, unpub. Master's thesis, 35 p . Carr, D. D . , 1973, Geometry and origin of oolite bodies in the St. Genevieve Limestone (Mississippian) in the Illinois Basin: Indiana Dept . Nat . Resources, Geol. Survey Bull. 48, 81 p . Choquette, P . W. , and R . P . Steinen , 1 980, Mississippian non-supratidal dolomite, Ste. Genevieve Limestone, Il linois Basin; evidence for mixed-water dolomitization: SEPM Spec. Pub. 28, p. 1 63- 1 96. Clifton, H . E . , R . E . Hunter, and R. L . Phillips, 1 97 1 , Depositional structures and processes in the non-barred high energy nearshore environment : Jour. Sed. Petrology, v. 4 1 , p. 65 1 -670. Cussey, R . , and G. M. Friedman , 1977, Patterns of porosity and cement in ooid reservoirs in Dogger (Middle Jurassic) of France: AAPG Bull . , v. 6 1 , p. 5 1 1 -5 1 8. Davies, D . K . , F. G. Ethridge, and R. R . Berg, 1 97 1 , Recognition of barrier en vironments : AAPG Bull . , v. 55, p . 550-565. Donaldson, J. A. , and B . D. Ricketts, 1 979, Beachrock in Proterozoic dolostone of the Belcher Islands, Northwest Territories, Canada: Jour . Sed. Petrology, v. 49, p. 1287-1294. Dunham, R. J . , 1970, Meniscus cement, in 0. Bricker, ed . , Carbonate cements: John Hopkins Univ . , Stud. Geology, no. 19, p. 297-300. Ferm, J. C . , et a!, 1 97 1 , Carboniferous depositional environments in northeast Kentucky: Ann. Spring Field Conf. , Geol. Soc. Kentucky Guidebook, 30 p . Flood, P . G . , 1 974, Sand movements on Heron Island; a vegetated sand cay, Great Barrier Reef province, Australia, in A. M . Cameron et a!, eds . : Brisbane, Australia, Proc . , 2nd Int . Coral Reef Symp . , v. 2, p. 387-394. Folk, R. L . , and A. S. Cotera, 1 970, Carbonate sand cays of Alacran reef, Yucatan , Mexico, in Sediments: Washington, D . C . , Smithsonian lnst . Atoll Research Bull . No. 1 37 , 16 p. __ and R. Robles, 1 964, Carbonate sands of Isla Perez, Alacran reef com plex, Yucatan : Jour. Geology, v. 72, p . 255-29 1 . Gerhard, L . C . , S . B . Anderson, and J . Berg, 1978, Mission Canyon porosity development, Glenburn Field , North Dakota, Williston basin : 24th Ann. Williston Basin Symp . , Mont. Geol . Soc. , p. 177-188. Hanford, C. R., 1 976, Sedimentology and diagenesis of the Monteagle Limestone (Upper Mississippian)-a high energy oolitic carbonate sequence in the
southern Appalachian Mountains : Baton Rouge, La. State Univ . , unpub. Ph . D . dissert . , 209 p. Hansen, A. R . , 1966, Reef trends of Mississippian Ratcliffe zone, northeast Montana and northwest North Dakota: AAPG Bull . , v. SO, p. 2260-2268. Harms, J. C . , 1 979, Primary sedimentary structures: Ann. Review Earth and Planetary Sci . , v. 7, p. 227-248. Harris, P . M., 1 979, Facies anatomy and diagenesis of a Bahamian ooid shoal : Miami, Sedimenta VII, Univ. Miami, Rosenstiel School of Marine and At mospheric Sci . , 1 63 p . Hayes, M . 0 . , 1 976, Transitional-coastal depositional environments, in M . 0. Hayes and T . W . Kana, eds . , Ter rigenous clastic depositional en vironments : AAPG Field Course, Univ. South Carolina Tech. Rept. No . 1 1 -CRD, p. I/32-I/ 1 1 1 . Hine, A. C . , 1 977, Lily Bank, Bahamas; case history of an active oolite sand shoal: Jour. Sed . Petrology, v. 47, p . 1554-1581. Hobday, D. K . , and J. C. Horne, 1 977, Tidally influenced barrier island and estuarine sedimentation in the Upper Carboniferous of southern West Virginia: Sed . Geology, v. 1 8 , p. 97- 122. Hubbard, D . K., G . Oertel, and D . Num medal , 1 979, The role of waves and tidal currents in the development of tidal-inlet sedimentary structures and sand body geometry; examples from North Carolina, South Carolina, and Georgia : Jour. Sed. Petrology, v. 49, p. 1073- 1 092. Inden, R. F . , 1972, Paleogeography, diagenesis, and paleohydrology of a Trinity Cretaceous carbonate beach se quence, central Texas: Baton Rouge, La. State Univ. , unpub. Ph . D . dissert . , 264 p . ___ , 1974, Lithofacies and depositional model for a Trinity Cretaceous se quence, central Texas, in B. F. Perkins, ed . , Aspects of Trinity division geology, geoscience and man, vol. 8: Baton Rouge, La. State Univ . , p . 37-52. __ and J . C . Horne, 1 973, Caliche soil horizons on oolitic shoals and car bonate mud mounds in Carboniferous (Neuman Limestone) of eastern Ken tucky (abs.): AAPG Bull . , v. 57, p.
785-786. ___
and H. Kohn, 1 979, Dolomitiza-
tion of offshore carbonate deposits in Hammett Shale, Lower Cretaceous, Texas (abs.): AAPG Bull . , v . 63 , p . 472. Kerr, R. S., 1 977, Facies, diagenesis and porosity development in a Lower Cretaceous bank complex, Edwards
Limestone, northcentral Texas, in D. G . Bebout and R. G. Loucks, eds . , Cretaceous carbonates of Texas and Mexico: Austin, Tex . , Bureau Econ . Geology Rept . No. 89, p. 138-167 . Kraft, J. C . , 1 978, Coastal stratigraphic sequences, in R. A. Davies, Jr., ed . , Coastal sedimentary environments : New York, Springer-Verlag Pub . , p. 361 -384. __ and J. J . Chacko, 1979, Lateral and vertical facies relations of trans gressive barrier: AAPG Bull . , v. 63 , p . 2 145-2 163. Kumar, N . , and J . E . Sanders , 1 974, In let sequences; a vertical succession of sedimentary structures and textures created by the lateral migration of tidal inlets: Sedimentology, v. 2 1 , p . 291-323. Land, C . B., Jr . , 1 972, Stratigraphy of Fox Hill Sandstone and associated for mations, Rock Springs uplift and Wam sutter arch area, Sweetwater County, Wyoming; a shoreline-estuary sand stone model for the Late Cretaceous: Quart. Colo . School Mines, v. 67, no. 2, 69 p. Laporte, L., 1 969, Recognition of a trans gressive carbonate sequence within an epeiric sea; Helderberg Group (Lower Devonian) of New York State: SEPM Spec . Pub. 14, p . 98-119. Lindsay, R. F . , and C. G. Kendall, 1 980, Depositional facies, diagenesis and reservoir character of the Mission Can yon Formation (Mississippian) of the Williston Basin at Little Knife field, North Dakota, in R. B. Halley and R. G. Loucks, eds . , Carbonate reser voir rocks: Denver , Colo . , Notes for SEPM Core Workshop No . l , p . 79- 1 04. Loreau, J. P . , and B. H. Purser, 1 973, Distribution and ultrastructure of Holocene ooids in the Persian Gulf, in B . H . Purser, ed. , The Persian Gulf: New York, Springer-Verlag Pub . , p . 279-328. Loucks , R. G . , 1977, Porosity develop ment and distribution in shoal-water carbonate complexes, subsurface Pear sall Formation (Lower Cretaceous) south Texas, in D. G . Bebout and R. G. Loucks, eds . , Cretaceous car bonates of Texas and Mexico: Austin, Tex . , Bureau Econ. Geol. Rept. No . 89. Lozo , F. E . , and F . L. Stricklin, 1956, Stratigraphic notes on the outcrop Basal Cretaceous, central Texas: Trans., Gulf Coast Assoc. Geol. Socs . , vol. VI, p . 67-78. Moore, C. H . , Jr. , J . H. Smitherman, and S. H. Allen, 1972, Pore system evolution in a Cretaceous carbonate beach sequence, in Stratigraphy and
Beach
sedimentology: Proc . , Sec. 6, Internat . Geol. Cong. , no. 24, p. 124- 1 36. ___, 1973, Intertidal carbonate cemen tation, Grand Cayman, West Indies: Jour . Sed . Petrology, v. 43, p. 59 1 -602. Multer, H. G . , and J. E. Hoffmeister, 1968, Subaerial laminated crusts of the Florida Keys: Geol. Soc. America Bull . , v. 79, p. 1 83- 1 92. ___, 1 97 1 , Holocene cementation on skeletal grains into beachrock, Dry Tor tugas, Florida, in 0. Bricker, ed . , Car bonate cements: John Hopkins Univ . , Studies i n Geology, no. 19, p . 25-26. Petta, T. J . , 1 977a, Diagenesis and paleo hydrology of a rudist reef complex (Cretaceous), Bandera County, Texas: Baton Rouge, La. State Univ . , unpub. Ph.D. dissert . , 212 p. __ , 1977b, Diagenesis and geochem istry of a Glen Rose patch reef com plex, Bandera County, Texas, in D. G. Bebout and R. G. Loucks, eds., Cretaceous carbonates of Texas and Mexico: Austin, Tex . , Bureau Econ . Geol. Rept . No. 89, p. 1 38-167. Purser, B . H., 1973, Sedimentation around bathymetric highs in the southern Persian Gulf, in B. H. Purser, ed . , The Persian Gulf: New York, Springer-Verlag Pub . , p. 157-178. __, 1 975, Tidal sediments and their evolution in the Bathonian carbonates of Burgundy, France, in R. N . Ginsburg, ed . , Tidal deposits: New York, Springer-Verlag Pub . , p.
335-343. and G. Evans, 1973, Regional sedimentation along the Trucial Coast , S. E. Persian Gulf, in B. H. Purser, ed . , The Persian Gulf: New York, Springer-Verlag Pub . , p. 2 1 1 -23 1 . Randazzo , A . F . , G . C . Stone, and H . C . Saroop, 1 977, Diagenesis of Middle and Upper Eocene carbonate shoreline se quences, central Florida: AAPG Bull. , v. 61 , p. 492-503. Roberts, H . H . , T . Whelan, and W . G. Smith, 1 977, Holocene sedimentation at Cape Sable, south Florida: Sed . Geology, v. 1 8, p. 25-60. Schwartz, R. K . , 1975, Nature and genesis of some storm washover deposits: U . S . Army Corps of Engineers, Tech . Memo . No. 6 1 , 69 p . Shelton, J . W . , 1973, Models o f sand and sandstone deposits; a methodology for determining sand genesis and trend : Norman, Okla. Geol. Survey Bull. No. 1 1 8 , 122 p. Shinn, E . A . , R. M. Lloyd, and R . N. Ginsburg, 1 969, Anatomy of a modern carbonate tidal flat , Andros Island, Bahamas: Jour. Sed. Petrology, v. 39, p. 1 202- 1 228. ___ , 1973, Sedimentary accretion along the leeward, S . E . coast of Qatar penin sula, Persian Gulf, in B. H. Purser, ed . , The Persian Gulf: New York, Springer-Verlag Pub . , p. 1 99-209. __ , 1982, Tidal flat environments in P . A . Scholle, D . G . Bebout, C . H . __
265
Moore, eds . , Carbonate depositional environments : AAPG Mem. 33, this volume. Stricklin, F. L . , Jr . , C. I. Smith, and F. E. Lozo, 1 97 1 , Stratigraphy of Lower Cretaceous Trinity deposits of central Texas: Austin, Tex . , Bur. Econ. Geol. , Rept . no . 7 1 , 63 p. ___ and ___ , 1972, Environmental construction of a carbonate beach com plex, Cow Creek (Lower Cretaceous) Formation of central Texas: Geol. Soc. America Bull . , v . 84, p. 1 349- 1 368. Taylor, J . C. M . , and L . V. llling, 1 97 1 , Development of Recent cemented layers within intertidal sand-flats, Qatar, Per sian Gulf, in Owen Bricker, ed . , Car bonate cements : John Hopkins Univ . , Studies in Geology, no . 19, p. 27-3 1 . ___ and ___ , 1 97 1 , Variation in Recent beachrock cements, Qatar, Per sian Gulf, in Owen Bricker, ed . , Car bonate cements : John Hopkins Univ . , Studies in Geology, no. 1 9 , p . 32-35. Thomas, G. E . , and R. P. Glaister, 1960, Facies and porosity relationships in some Mississippian carbonate cycles of Western Canada basin : AAPG Bull . , v. 44, p. 569-588. Wittstrom, M . D., Jr., and M . E . Hage meier, 1978, A review of Little Knife field development, North Dakota: 24th Ann. Williston Basin Symp., Mont. Geol. Soc . , p. 361-368.
268
Shelf
G RAINY S E Q U ENCE
M U D DY S E Q U ENCE FLAT PEBB LE BRECCIA
/
/
THI N LAMI N ATIONS
__..- OCCASION AL BURROWS
c
DESSICATIO N POLYGON S
-----
---- E VEN
TO IRREGULAR LAMINATIONS + FENESTRAE
---- SKELETAL
D
C
N O N -SKELETAL LIME SAND
LIME
SAND
B
SKELETAL OR
HARDGROUND
� ---- FOSSILIFEROUS, B U RROWED + BIOTURBATED LIME M UDSTONE
B
---
------ ARGILLACEOUS ---- LIMESTO N E
A
SKELETAL + INTRACLAST --- LIME SAND OR CONGLOMERA TE
R
estricted shelf, bay, or la goon environments can be reasonably well defined ge ographically or hydro graphically in modern settings. In the geologic record, these terms are typically used somewhat more loosely to form a "waste-basket" category for low-energy, shallow-water car bonates, especially if a demonstrable barrier occurs seaward , or if lateral facies equivalents reflect severe restriction, as in the case o f evaporitic or euxinic (organic carbon-rich) se quences. Geographically, bays and lagoons are partly enclosed bodies of water. A bay is a shoreline recess or inlet be tween headlands. Headlands in car bonate settings may be formed by depositional topography from the current depositional cycle as well as by erosional or depositional topography from a previous cycle. Lagoons are more completely en closed (for example by barrier islands or reefs) so that connection to the sea is distinctly restricted . Lagoons are normally shallow, although lagoons
Figure 1 -Hypothetical shoaling-upward carbonate sequences of relatively low energy (left) and somewhat higher energy (right) (from James, 1 977). Let ters ind icate: A. Basal transgressive lag; B . Subtida l , restricted to open marine; C. I ntertidal; D. Supratidal.
of some larger atolls reach 70 m deep; bays have no particular depth connotation. Restricted shelves may be defined as any part of a continental or island shelf with slow water circulation resulting in abnormal salinity, depleted nutrients, or temperature ex tremes. Restriction that reduces nor mal wave or current energy may result from any physical barrier such as reefs , islands, skeletal or oolitic sand shoals, or from the damping ef fect of vast expanses of shallow water . Abnormal salinities resulting from restriction may be elevated , diluted, or seasonally fluctuating depending on climate and access of terrestrial runoff. Restricted waters are commonly subject to temperature extremes, both high and low, especially if shallow, and to oxygen and nutrient depletion. In ancient settings, restricted shelf, bay, or lagoonal depositional en vironments may be inferred from stratigraphic evidence of paleo geography similar to that described for modern settings, for example by
transition to reefal or sand-shoal (ooid, skeletal, terrigenous) deposits on the one hand and to shoreline, tidal flat, or evaporitic facies on the other. With less stratigraphic resolu tion , restriction may be inferred from impoverishment of fauna or from lithofacies, such as muddy sediment, especially if it contains abundant organic matter , pyrite, or evaporites. Summary of Diagnostic Criteria
No single criterion can be diagnostic for environments as diverse as restricted shelves , bays, and lagoons. Sequence and transitions are the most reliable guides. Lateral tran sition to a barrier facies in one or more directions - for example to a reef, barrier bar/island , or oolite shoal - coupled with transitions to low-energy shoreline facies in other directions is typical . Some possible transitions are outlined in Table 1 . Vertical sequence can be used to in-
Shel'
269
TAB LE 1 Seaward or Windward Environment
Landward or Leeward Environment
E nvironment
Example (Age )/Reference
E nvironment
Example (Age)/Reference
Reef
Belize Shelf (Holocene); Wantland & Pusey, 1 975 El Abra Fm. (m. Cretaceous) ; Carrillo. 1 971
lidal Flats
Great Bahama Bank. Andros Lobe (Holocene); Shinn & others, 1969 Black River Grp. (Ordovician); Walker. 1972
Salt Flats (Sabkha)
Persian Gull (Holocene); Purser. 1 973 Edwards Grp. ; ( L. Cretaceous); Rose, 1 972
Evaporites
E l Abra Fm. (mid Cretaceous); Carrillo. 1 971
Euxinic (Shelf Basin)
McKnight Fm. , Edwards Grp. (Lower Cretaceous); Rose, 1 972
Swamp ( Peat, Coal)
Florida Bay, Everglades (Holocene) ; Enos & Perkins, 1 979
Beach
Cow Creek Mbr., Pearsall Fm. (Lower Cretaceous); Loucks, 1977 Great Bahama Bank, Eleuthera Lobe (Holocene) Campeche Bank & Yucatan (Holocene)
Oolite Shoal
Little Bahama Bank (Holocene) ; Hine, 1977 Smackover Fm. (Jurassic); Wilson. 1 975
Skeletal Sand Shoal
Florida Shelf Margin (Holocene); E nos. 1 977
Barrier Island
No. Biscayne Bay, Fla. ( Holocene); Wanless, 1969
So. Biscayne Bay, Aa. (Holocene); Wanless, 1969 West Florida Bay (Holocene); Enos & Perkins, 1 979
Mud Bank
Florida Bay (Holocene); Enos & Perkins, 1 979 Great Bahama Bank (Holocene); Enos. 1 974b Shark Bay, W. Austr. (Holocene ) ; Logan & others, 1 970
Island ( Prior topography)
c:
g
j
�
Erosional Coast Karst Plain
Table 1 . Typical lateral transitions for restncted shelf. ba y . and lagoon en,wonmeflts.
fer lateral relat i o n s . A t ypical ver t i c al sequence is a lo w-energy shoaling upward cycle ( Fi g .
I)
consisting o f a
basal t ransgressive u n i t , m uddy car
( Laporte,
1 967)
o f more o r less ran
dom t ransitions among m uddy, low
rant growth forms are also c l ues t o
ad'> erse cond i t io n s . E. x t n: m c ad\ersi t \'
energy l i t ho facies and/or im poverish
may result i n few organ ic rem a i n s o f
ed organic co m m un i t ies . Such t ran si
any k i n d .
An abundance o f burrow.'> i s c h a r
bonate with impoverished fauna, and
t ions are generated readily i n very
capped by i ntertidal and/or suprat idal
shallow water where m inor changes in
acteri s t i c , alt hough cert ainly not
sea level or depmitional t opography
d i agnos t i c , of restricted subt idal
deposi ts. Although t hey have not
deposi t s . Burrows may he of one or a
received equi valent notice i n recent
can cause signi ficant changes in en
compi lations of depositional mode l s ,
vi ronment . In cont rast . �orne expan
few dist inctive t ypes . i n dica t i ve of a
many cycles c o n t a i n i n g restricted in
sive set t i ngs with l i t t l e relief. such as
restr i ct ed fa u n a , or the deposits may
tervals are t ransgressive or include
the i n terior of the modern Great
have an indistinct mot t led o r swirled
transgressive i ntervals. I n these,
Bahama Ban k , develop monotonous
appearance i n dicat ing extensi v e
disconform i t y 'i u r faces, t i dal-flat
m uddy facies t h at may extend for
biot u rbation . The e n d member i s
deposits, or shoreline facies are
h u ndreds of k i lometers laterally or
complete homogen izat i on o f t h e
overlain by low-energy, semirest ricted depo5its a n d i n t ur n by
for many meters vertical l y .
deposits w i t h loss o f defi n i t ion of in
higher energy, more open-marine
i mport ant clue t o restricted set t i ng s .
d i re�..:� evi dence o f burrowing may be
deposits.
Rest ricted set t i ngs typical ly h a v e low
lost . Not all struc t u res in shaliO\\
In
Depauperate biot a is a commo n ,
dividual burro w s , so t hat . ironical l y .
faunal d i versity, but relatively great
rest ricted environments are biogenic,
quences or cycles , rocks from
num bers of t h ose few organ isms
however . I n set t i ngs where macroben
restricted environments may be c h ar
adapted for survival . Reduced size of
t hos are se\'erely reduced i n n u m ber .
the absence o f wel l-d efined se
act erized by
a
" facie� mo�a i c "
i n d i v i d uals ( d warf faunas) and aber-
or la-: k in g , l am i n at ion may he
270
Shelf
'v ,
Figure 2a-Modern serni-reslricte-0 set· lings; Florida Bay, restricted by Pl�isto��r.e coralline limeston9 forming tht> Florida Keys {�fter Enoo artd Pa rkiM, 1 979).
Rgure 2b-Modern semi·restrlc�ed .set· tin� s : The Bahama Banks showing areas of maxim um ¥astriction (stippl ad).
·:.::..=��Ul'l NUb. 0� m"J.�'In::
i
w.a
Shelf
- SHELLS
AND FRAGMENTS
/S *' INTRACLASTS
- BIOT U RBATION - BURROWS
271
- GRASS
� PLE ISTOCENE � ROCK FLOOR
Figure 3-Diagrammatic view of Florida Bay mud bank. Vertical exaggeration is about 1 00x.
preserved. Fine millimeter-scale
SETTING
lamination characterizes deposits of anaerobic settings (Byers, 1 977). In
hypersaJine settings, lamination or
Lateral Facies Relationships
Lateral facies relations may be the
nearer the slope break (Enos, 1 977). The transition from cross-laminated skeletal sand to burrowed muddy
sediments is abrupt (Swinchatt, 1 965),
bedding is commonly distorted by in
most reliable clue in recognition of
especially where there is a marked
terstitial crystal growth, dehydration
restricted shelf, lagoon, or bay
shrinkage, or solution of evaporites.
deposits in the rock record . Seaward
change of relief from the sand shoals ( 1 to 4 m water depth) to level bottom muds (5 to 1 5 m deep).
Non-skeletal grain composition is
or windward equivalent deposits in
not very diagnostic for recognition of semi-restricted shallow-water en vironments, but fecal pellets and
many instances produce the restric tions, but landward or leeward equivalents may record the restricted
peloids of diverse origins are charac
conditions more drammatically. Both
teristic sedimentary particles; locally they may comprise virtually the entire sediment. "Grapestone" aggregates and small intracl ast s are also common constituents. Mineralogy is scarcely indicative of depositional environment, not even i f the metastable sedimentary miner alogic assemblages were preserved .
produce more distinctive facies than the commonly nondescript rocks representing semi-restricted subtidal environments. Laterally equivalent facies repre senting more open water conditions (Table I) include reef complexes and sand shoals consisting of ooids, skeletal grains or even quartz. For ex
However , evaporites and penecontem
ample, the inner part o f the south
Islands formed by depositional or erosional t opograp h y of contem poraneous or earlier depositional episodes may also contribute to restriction . Islands formed by sedi ment accretion such as the sand and rubble cays associated with barrier bars or reefs are actually only a minor part of the restricting mass; submarine shoals or reef growth pro duce the major bulk. Islands may form from erosional remnants of older rocks or from eolian dunes, sand shoals, or reefs of a previous depositional episode. Examples of
poraneous dolomites are common in
Florida shelf margin (Fig. 2a) is a
and lagoons or in laterally equivalent
within 5 km of a major slope break .
by pre-existing depositional
deposits.
The immediate cause of restriction is
topography are the upper Florida
more restricted portions o f shelves
semi-restricted , low-energy shel f
circulation-restricting islands formed
skeletal sand shoals, although these
Keys, consisting of slightly eroded
are probably directly dependent on
Pleistocene coral reefs that seal off one fl an k of Florida Bay (Fig. 2a; Enos and Perkins, 1 979) ; the lower
the skeletal productivity and wave
transport of skeletal grains from reefs
272
Shelf
Figure 4-Piastic cast of burrow ex cavated by the ghost shrimp Callianassa.
Photograp_h �ourtesy E._ A. Shinn.
Florida Keys , consisting of Pleistocene oolite bars (Perkins, 1 977) with leeward carbonate banks (Jin drich, 1 969; Basan , 1 973); the major Bahaman islands, consisting of Pleistocene eolianite ridges leeward of which are the most restricted portions of t he Bahama Banks (Fig. 2b, Enos, 1 974b) ; and Bermuda, also consisting of Pleistocene eolianite dunes that enclose Harrington Sound (Neumann , 1 965) and Great Sound - semi restricted environments in an open oceanic sett ing Because the margins of carbonate platforms are typically elevated, whether from karstic ero sional processes (Purdy, l 974a) or .
from differential accumulation of car panses of shallow water , such as ex bonates, rocky islands may have caus isted in epeiric seas, can damp out ed restriction in many ancient set tidal and wave energy. Modification tings. Recognition of the existence of of the water mass may follow in the a paleo-island, which involves lateral form of hypersalinity from prolonged transition to a non-depositional set evaporation, dilution from rainfall ting, is often more difficult than and runoff, or salinity fluctuations recognizing a facies transition, from strongly seasonal or variable especially given the facies complex climates. Shoal waters have large ities o f a carbonate shelf margin. The temperature fluctuations in response criteria for recognizing disconformity to air temperature changes. Long surfaces are now receiving attention residence time of water on shallow (Perkins, 1 977; Read and Grover, shelves, because of poor circulation, 1 977; Esteban and Klappa, 1982). may lead to severe nutrient depletion. Restricted settings may also Facies patterns in idealized epeiric originate without significant seas have been considered by Shaw physiographic barriers . Broad ex( 1 964) and Irwin ( 1 965). Basically,
Shelf
273
Fig u re 5-Peloidal sedimen1 from the Great Bahama Ba n k, including both
hardened (da rk) and soft peloids (light brown with indisti nct outlines). Definition of soft peloids was enhanced by drying during sample preparation . Scale bar is 0.3 mm. Photog raph cou rtesy of R. D . Perkins.
low-energy, muddy carbonates (or mixed t errigenous muds) are deposited in more open, deeper water. Higher energy, winnowed sediments accumulate as the bottom slope intersects wave base, a geometry termed the carbonate ramp by Ahr ( 1 973). Very shallow-water, restricted environments develop further leeward or landward and may cover vast ex panses with little facies differentia tion. Lateral transitions from such low-relief restricted settings are likely to be very gradual and to fluctuate in position with time. Landward facies changes from restricted shelves and lagoons are
from subtidal settings to shoreline en vironments . Shoreline facies may be narrow belts of beach-complex deposits which pass in turn into ter restrial deposits (deltaic or fluvial) or non-depositional surfaces (a karst plane) . Higher-energy facies, typically shelly sands, may be developed at the shoreline by focusing of local waves that develop even within restricted lagoons. More typical shoreline transitions are from restricted subtidal settings to tidal-flat complexes. These very distinctive deposits (Shinn, 1 982) may include sediments ranging from humid salt marshes to sabkhas with hypersaline interstitial waters. Typically subtidal and tidal-flat facies are extensively interbedded through many vertical and lateral transitions. Well-documented examples occur in the Manlius Formation (Lower Devo nian) of New York (Laporte, 1 967)
and in the Lofer cycles (Upper Triassic of the Alps, see below; Fischer, 1 964). Although there are apparently no documented examples of lateral tran sitions from limestone to coal, several modern sub-tropical carbonate lagoons are bordered by mangrove swamps where peat is actively ac cumulating. A notable example is Florida Bay and the adjacent Everglades (Spackman et al, 1 964; Enos and Perkins, 1 979). Depositional Geometry
Depositional geometry of restricted carbonate settings may range from simple level-bottom expanses ex emplified in the Holocene by the Great Bahama Bank, to complex, compartmentalized environments where depositional relief has developed, as in Florida Bay (Ginsburg , 1 956; Enos and Perkins,
274
Shelf
Fi g u re 6-Grapestone from the Great Bahama Bank. Cross-polarized light. Largest grapestones are about 3 mm in maximum dimension . Photograph courtesy of R. D. Perkins.
Figure 7a-Requienid rudists: a Cretaceous bivalve roughly analogous to oysters in life style. Cl uster of r equie n ids in l ime packstone with peloids and mi l iolid 'foraminifers (white specks). El Abra Fo rmat i on {mid-Cretaceous). Sierra de El Abra, Sa n Luis Potosi, Mexico. Slab is 1 2 em wide. Sample courtesy of C. J. M inero.
1 979), the coastal lagoons of the north coast of Cuba (Price , 1 967), and the Belize lagoon (Wantland and Pusey, 1975). Compartmentalized set tings may produce rapid lateral and vertical facies transitions which ap pear random in detail, aptly termed " facies mosaics" by Laporte ( 1967). Large-scale progradational or trans gressive patterns may be recognizable despite the small-scale complexity (Laporte, 1 969) . In contrast , the monotonous level bottom facies so typical of epeiric-sea deposits might be called " facies pro saic. " The Ellenburger Group (Lower Ordovician) of Texas is an example (Cloud and Barnes, 1 956). Similar sheet-like geometries develop in the interior of large isolated banks or platforms that may have precipitous sides with sharp facies gradients , but broad flat interiors. Examples are the modern Great Bahama Bank, the mid-Cretaceous (AIbian-Cenomanian) Valles-San Luis Potosi and Golden
Lane platforms of central Mexico (Carrillo, 1 97 1 ; Enos, 1974a), and the Middle Permian of the northwest shelf of the Permian basin in Texas and New Mexico (Meissner, 1 972) . These units are tens to hundreds of kilometers wide and cover thousands of square kilometers with very little interior facies differentiation. Typical lithofacies are lime mudstones, mud dy peloidal packstone and wackestones , and foraminiferal (miliolid, fusulinid) wackestone. On isolated platforms surrounded by deep water, islands and tidal-flat complexes typically develop at the raised platform margins, leaving slightly deeper water in the interiors without much facies differentiation. If relief is developed all around the platform , evaporites may develop in the interior, as in the mid-Cretaceous platforms of Mexico (Viniegra, 1 97 1 ) . In the modern , the raised plat form margin of the Bahama Banks is much more pronounced on the windward
Shelf
Figure 8 - Poo r l y sorted packstone that con ta ins large fragments of oysters (dark shells) and serpulid worm tubes (wh ite). Lower part of Bexar Shale Member, Pearsa l l Formation (Lower Cretaceous). Med i n a County, south Texas. Slab of core. 8 em wide. from Tenneco No. 1 Ney. From Loucks (1 977); photog raph cou rtesy of R. G. Loucks.
Figu re ro-Requienid
Crelaceous bivall/9 ovsters in life -s tyle.
rudists: a
rotJQhly analogous to R equ i&nid lime wackestone. Whole and broken shells m a matrix Q f lime mud c ut bv horsetail
styloli tes (prommen� at tC?p'and bottom). Lower pa rt of Bexar Shale Member. Pearsal l :=:ormation (lower Cretaceous). M adina County, sou1h Texas. S lab of core, 8 ern wi de , f!"OITI Tennt:co No. 1 NfJy. Frorn liJucks (1977); phorograph courtesy of R. G. Loucks.
0
275
CM
5
IN
2
Figure 9a-Burrowed and bioturbated
lithologies. Bioturbated Holocene sedi ment from a Florida Bay mud bank. Depositional texture corresponds to a pelleta l-mol luskan l ime wackestone. Ver tical t ubes were formed by grass roots. Core slab. impregnated by blue plastic (from Enos and Perkins, 1 979).
20o/o above normal seawater , far short of the 4-fold concentration necessary for the onset of the evaporite precipitation . The facies pattern is a simple gradation from soft pellet mud in the most restricted areas to hard pellet sand toward the
leeward bank margins. In the areas of margins (Fig. 2b) , giving rise to asym metric facies distribution in the lee, with maximum restrictions and mud
slow deposition the pellets are bound
together in "grapestone" aggregates . With the development of embayed
diest sediments adjacent to the large windward islands (Smith , 1940). However, in the present semi-humid,
tion through depositional relief, the
subtropical climate of the Bahamas, salinities reach only about 42 ppt,
more complex. Florida Bay is a familiar modern example (Fig. 2a) .
shorelines and/or compartmentaliza depositional geometry becomes much
276
Shelf
Fig u re 9b-Bu rrowed and bioturbated lithologies. Bu rrow-mottl ed ostracod foramin ifer lime wackestone. Bu rrows are filled with clean peloid miliolid lime packstone. Structure at lower right is a soil pipe from an immediately overlying subaerial erosion surface. El Abra Formation (mid-Cretaceous), Sierra de El Abra, San Luis Potos i Mexico. Slab is 9 em wide. Sample courtesy C. J. M i nero. ,
The low-relief Florida mainland forms the northern side of the triangular bay. The southeastern side is enclosed by a ridge of Pleistocene coralline limestone and the southwestern limit is formed by depositional relief of broad mud banks, developed in the Holocene, rising to sea level . The interior of the bay is compartmentalized into shallow basins by anastomosing linear mudbanks . Mud is winnowed from the basins by periodic storm waves and trapped on the shallow grass covered banks. The result is a basal shelly lag in the basins and muddy banks which contain virtually all of the sediment . The relief of the banks is very small scale (about 2 m) with gentle slopes ( < 1 °), but nevertheless produces more facies differentiation (Fig . 3). The narrow windward margins of the banks comprise shelly packstone; however, the bulk of the sediment is bioturbated pelletal molluscan wackestone with local pockets of laminated pellet mudstone or very fine-grained packstone. These pockets result from the filling of negative micro-relief, probably ero sional scours, as well as tractional deposition of soft, low-density pellets in low-amplitude dunes (Enos and Perkins, 1 979). Islands with supratidal algal flats and mangrove forests dot the mudbanks. The
resulting sediments are lenses of stromatolitic, mud-cracked pellet wackestone and organic-rich lime mud or peat . Local higher energy shelly lenses develop as island beaches , as lag deposits and sand waves in channels cutting the mud banks , and as low-relief spillovers spread from the windward margins onto the bank tops by storm currents . In more open settings at the seaward margins of the bay, bank edges act as miniature shelf breaks populated by a shoal-fringe community characterized by finger-corals . The resultant sedi ment is a muddy skeletal gravel . Tidal deltas, developed on the channels into Florida Bay and Bis cayne Bay, have most of the facies characteristic of Florida Bay mud banks (Ebanks and Bubb, 1 975). The overall geometry is, of course, much different ; the triangular plan and wedge-shaped cross section of classical deltas are developed. Other facies packages within the generally low-energy delta complex are associated with the tidal channels, ac tive and abandoned, and the natural levees which may have mangrove forests and islands . The seaward margins of deltas also form mini-shelf breaks with the development of the shoal-fringe finger-coral community and coarser sediment (Enos, 1 977) .
Figu re 9c-Burrowed and bioturbated lithologies. Burrowed packstone contain ing m i liolid foraminifers, echinoids, and mollusks. Pearsall Formation (Lower Cretaceous), south Texas. Slab of core is 8 em wide . Photograph courtesy of R. G. Loucks.
Other Modern Settings
Other modern semi-restricted car bonate environments apparently fall between the extremes of the level bottomed Bahama Banks and com partmentalized Florida Bay, although 3-dimensional data are generally sparse. Patterns of depositional relief, however , vary considerably. In the somewhat less restricted set ting of the inner part of the south Florida shelf margin, depositional relief is more subdued, but still exerts important influence on facies pat terns. Linear anastomosing mudbanks are lacking, but a few broad , low relief banks have developed . Rodri quez Key bank is the best known (Turmel and Swanson, 1 976) . It is a flat-topped mudbank elongate transverse to depositional strike with
Shelf
Figure 1 0a-Carbonate cycles in the Cambrian oi the Canadian Rocky Moun tai ns. Transition zone from Arctomys Formation (below. M i ddl e Cambrian) to Watertowl Formation (Upper Cambrian) ; Chaba River section on Al berta-British Columbia boundary. Cycles delineated by color contrasts consist of dark. thin· bedded lime mudstone grading upward into yellow-weathering dolom ite crypt alga! laminite. Cross-laminated dolomitic siltstones at the top of the dolomite interval c.'O ntain calcite IJieiJs which may have replaced evaporites. Photograph by J . 0. Aitken. a well-developed seaward shoal-fringe community producing a linear bank of skeletal gravel. The finger corals are joined in this less restricted setting by branching red algae and plate producing green algae. The muddy, grass-covered bank interior is a favorite haunt of burrowing ghost shrimp who have converted it into a monument to burrow architecture
(Shinn, 1 968b). A small mangrove island near the landward edge of the bank has produced a peat lens. A storm ridge of skeletal sand has developed on an analogous mangrove island on neighboring Tavernier bank . Other slightly deeper muddy banks which dot the Florida inner shelf margin have developed patch reef caps; one has developed into a shoal of rippled skeletal sand sur rounded by level-bottom muds (Enos, 1 977). A much larger mass of muddy sediment with little facies differentia tion is contained in low-relief wedges which have accreted seaward from the Florida Keys. These range from wedges with relief of a few decimeters along embayments in the shoreline to a broad wedge extending several kilometers seaward with relief of 5 m or more (Enos, 1 977) . The Gulf of Batabano , a large em-
277
bayment on the south coast of Cuba, is partially blocked by reef barriers , but salinity is near normal throughout . Surface sediment in near ly half the bay contains more than 50% mud ( < 62J.Lm, Daetwyler and Kidwell , 1 959). Several muddy shoals as much as 50 km long and several kilometers wide snake across the em bayment. There are, unfortunately, no data on sediment thickness and it appears possible that the shoals may reflect structure in the underlying rock. The Belize lagoon is enclosed be tween the mainland of the Yucatan Peninsula and the longest barrier reef in 'the Western Hemisphere, but salinities do not rise above normal marine values (Wantland and Pusey, 1 975) . Abundant runoff from adja cent highlands results in brackish rather than elevated salinities in the
278
Shelf
Figure 1 Ob-Carbonate cycles in the Cambrian of the Canadian Rocky Moun tains. Lyell Formation (U�p�r Cambrian), Mount Ogden, Yoho National Park, British Columbia. Cycle (delineated by
ticks) begins with erosive base, overlain
by laminite clasts and by dark, no d u l ar lime mudstone with microcrystalline do lom i te partings. The top of the cycle is
dolomite c ryptalgal laminite (Aitkin, 1 978). Photog raph courtesy of J. D. Aitken.
nearshore parts of the lagoon and a lagoon floor blanketed with mont morillonitic mud. Terrigenous muds grade seaward into calcareous mud derived from the reef tract. The seaward and southern portions of the
lagoon are studded with steeped sided "pinnacle" reefs and micro-atolls or " faroes," (Purdy, 1 974b) which grow on pinnacles in Pleistocene rock that in turn reflect deeper structure. Thus Holocene depositional relief here merely amplifies previous relief o f
possible karstic origin (Purdy, 1 974b). Shark Bay, Western Australia, is a modern example o f a hypersaline lagoon with salinities reaching 70 ppt , twice normal sea water (Logan et a!, 1 970). The bay is cut off from the open Indian Ocean (except at the north end) by ridges of Pleistocene calcareous eolianite. Although the bay lies in the belt of southeast tradewinds and has a considerable fetch over its area of 1 2 ,000 sq km, the northward opening keeps waves subordinate to tides in moving water masses. Sediments on the flat floor of the bay are sand-to-silt-sized skeletal fragments ( foramin iferal-mo ll uscan microcoquinas). Around the shallow margin' of the bay, the skeletal sediments are diluted by quartz sand and oo ids . The bulk of the sediment in the bay has accumulated in a grass covered bank that extends along the mainland at the northeast margin of
the bay (Davies, 1 970). The bank is a wedge-shaped deposit 1 30 km long, about 8 km wide, and 9 m thick at its seaward edge. The seaward edge is less than 2 m deep and the bank gradually shoals into an extensive tidal flat. Numerous tidal channels with well-developed levees cut the bank. Bank and levee sediment con tains about 300Jo mud ( < 621-!m), con sisting of foraminifers and red-algae that encrusted the sea grass . Sand sized particles are foraminifers (about 500Jo), mollusks (about 250Jo), cor alline algae, peloids, and quartz sand (Davies, 1 970) . Channel and intertidal sediments contain few fines and
relatively more quartz. Tidal-flat sediments have prograded over the subtidal bank sediments at its land ward margin to produce a shoaling and coarsening-upward sequence. The Persian Gulf is also hyper saline (40 to 45 ppt, in the open gulf)
Shelf
279
Channel la9 Collapse
breccia
Stromatolitic
SUPRATIDAL
hard
crusts
ond disrupted g y psum bearing zones
INTERTIDAL
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and pel l eted
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INTERTIDAL TO SHALLOW MARINE RESTRICTED C IRCU LATION
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SlJIRATIDAL
with a moderately high-energy car bonate province along the north facing portions of the Arabian coast (Purser, 1 973). The gulf floor slopes gently northeastward to form a trough more than 80 m deep along the Iranian coast that receives ter rigenous sediment from the Zagros Mountains in Iran and the Tigris Euphrates delta at the north end of the gulf. The coastal zone along the shallow Arabian side of the Gulf is a complex of sabkhas, small tidal lagoons, channels, ooid bars and beaches, and reefs, that are discussed elsewhere in this volume. The off shore sediments are a more orderly blanket of skeletal sand with a gradual seaward increase in the con tent of mud, both carbonate and ter rigenous (Houbolt, 1 957; Purser, 1973). Virtually the only relief on this broad shelf results from actively ris ing salt domes which produce small shoals of winnowed skeletal sand and/or living reefs . Depositional relief characterizes and profoundly influences so many modern muddy carbonate settings that it appears likely to have been a factor in many restricted ancient set tings. The gentle relief associated with mudbanks is difficult to document in ancient rocks , however . Most
Col l a pse
and m icrite
and
pe l l et&d
micr ite 109 breccio
recognized banks and mounds from the rock record are steeper sided organic buildups with well-developed flank facies . In view of the abun dance of mudbanks and comparable low-relief features in modern en vironments, it seems that many an cient carbonate sequences may have been influenced by penecontem poraneous depositional relief, par ticularly those in which facies mosaics are well developed . Microfacies
Facies of modern and ancient restricted subtidal settings are charac terized by a limited number of grain types: carbonate or terrigenous mud, fecal pellets, peloids, grapestones, in traclasts, and a limited range of skeletal components . Peloids (Fig. 5) are ovoid micritic grains of various origins such as micritication of skeletal and oolitic grains; biological , including fecal , aggregation of lime mud; and mechanical erosion of car bonate mud or lithified mudstone. In areas of slow deposition , grains of all these origins converge in appearance through rounding, micritization, and hardening (intra-particle cementation), so that the mode of origin is commonly indeterminate. Grapestones (Fig. 6) are clusters of
Figure 1 1 -Typical cycle representing restricted environments from the Ed· wards Group (Lower Cretaceous) of cen tral Texas (from Rose, 1 972).
carbonate sand grains bound together by encrusting foraminifers , filamen tous algae, and/or micritic cement (Windland and Matthews, 1 974). The resultant clusters are generally flakes as much as a centimeter in diameter with spongy, porous interiors. " En crusted lumps , " "constituent grains , " "organic aggregates ," "aggregates, " and "lumps" of various authors are virtually synonymous . Such grains are typical of shallow-water settings with slow sedimentation rates . Charac teristic skeletal components (Table 2) are benthic foraminifers (for example, miliolids, Fig. 7a and 9a; fusulinids); ostracods; gastropods, particularly high-spired types; bivalves such as oysters and requienid rudists (Fig. 7) ; algal oncoids; serpulid worms (Fig. 8); and lingulid brachiopods. Mechanical abrasion , reworking, rounding, and sorting are at a minimum, reflecting low-energy depositional environments. In con trast , micritization, boring, metal oxide st aini ng encrustation, and other indicators of slow accumulation ,
rates are generally extensive.
Wilson ( 1 975) listed the following standard microfacies (SMF) types as typical of restricted carbonate en vironments: peloidal grainstones (Fig. 5), locally with ostracods or
280
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FA C I ES
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281
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- l ight, gray-green carbonate sand with varying carbonate mud composition, cerith ids, b i va l ves and gypsum disks,
dolomitized
Fig u re 1 2-ldeal ized cycle of Holocene sediments beneath the Sabkha at Abu Dhabi, adjacent to the Persian Gulf. Units 2, 3. a n d 4 were deposited primari· ly in restricted subtidal environments. Compiled from various sources by McKenzie and others (1 980).
FACIES
Table 2 . Summary of some taxa com· mon in restricted shelf, bay, lagoon en vironments. (A) Group or genus which has broader ecologic range but is com· mon in restricted environments is enclosed in parentheses. (B) Long ranging groups are shown by horizontal lines; the name is entered and footnoted under the geologic period where specific references have been found. Ranges in· dicated are from Treatise on I n ·
vertebrate Paleontology, R. C. Moore, ed.
' Environmental range has become rest r i cted by compettlion i n Phane rozoic ti m e so that 'stromatolites· have become vtrtually synonymous with tidal-flat dtsposition. Blue-green a l gae persist t n a variety o f marine envt ronments although m icrittza · tion of grains may be their only recognrzable geologic record. 'Perktns, 1 974 'Loucks. 1 97 7 . •Wood and Wolfe, 1 959. 'Frscher, 1 964. 'Bosellini and Rossi, 1 974 . 'Newell and o th e r s , 1 953. 'Edte, 1 958. 'Laporte, 1 967. "Wilson, 1 967. "Zenger, 1 97 1 . "Walker, 1972. "Cloud and Barnes. 1 956. "Conrad, 1 977. "Fiugel, 1 97 7 . "Heckel a n d Baesemann, 1 975. "Tsten and Dricot, 1 977. "Heckel, 1 973.
foraminifers ( SMF 16); grapestone-peloid-intraclast grainstone (SMF 1 7 , Fig. 6); foraminiferal or dasyclad algal grainstone with peloids (SMF 1 8) ; laminated to bioturbated pellet mudstone or wackestone (Fig. 9) , locally with fenestral fabric (SMF 1 9) ; oncoid wackestone o r floatstone (SMF 22), and homogeneous un fossiliferous mudstone which may have gypsum crystals or casts (SMF 23). These are only a sampling of the many combinations of composition and textures from the components mentioned above. The variation in characteristic skeletal components with geologic age (Table 2) greatly ex pands the possibilities . This is true despite the fact that the relatively primitive, hardy forms able to cope with restricted environments are among the most persistent facies fossils known. One suite of facies not commonly ascribed to restricted lagoons, shelves, and bays are euxinic or organic-rich carbonates . These restricted en vironments can develop as brackish-
water, anoxic swamps, such as modern mangrove swamps, or as hypersaline shelf basins (Rose, 1 972). Their apparent scarcity in the geologic record may be in part due to oversight, although abundant fresh water and organic-rich , acid producing sediments are certainly not the most compatible settings for car bonate accumulation. A typical lithofacies is black, laminated or slightly burrowed lime mudstone, rich in organic matter , containing few or specialized fossils such as ostracods , lingulid brachiopods, or thin-shelled bivalves . Sequences
Far more diagnostic than individual lithofacies are associations of lithofacies, specifically the vertical succession in which they are ordered. As Johannes Walther noted long age (Walther, 1 894) , the vertical sequence of facies in the rock record reflects lateral sequence in the depositional setting , a principle that has become known as Walther's Law. This princi ple is especially important in subsur-
282
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e
Dolomitic algal mats 8 crusts; mud cracks V desiccation p o r e s ;
very restricted b i ota
BaS
Figure 1 3-Diagrammatlc representation of the " Lofer cyclothem , ' ) (Upper Triassic) Calcareous Alps. U nits a re desc ribed in text (from Fischer, 1 964).
face exploration with limited well data where it has found many ap plications. The principle is also ex tremely useful in carbonate rocks from restricted settings in that the lateral equivalents tend to be more distinctive than the restricted facies themselves . Considerable attention has been devoted to shoaling-upward sequences deposited on carbonate shelves (James , 1 977; Wilson, 1 975). A typi cal sequence (Fig. 1) begins with a "basal transgressive lag" representing a relative rise in sea level that is more
rapid than accumulation rates. The overlying unit commonly represents the deepest water and lowest energy in the sequence. Vertical accretion or lateral progradation produces shoal ing which may eventually reach the intertidal zone. Further accretion leads to supratidal sedimentation with possible subaerial alteration of the se quence . The actual elements present in a given sequence vary considerably, depending in part on whether shoal ing leads to dampening or focusing of energy . Thus the shallower portions of a cycle may be beach deposits, re-
fleeting relatively high energy, or ti dal-flat deposits where energy is generally very low. James ( 1 977) termed these examples "grainy se quences" and "muddy sequences , " respectively. This terminology is apt, as it recognizes that designations such as " fining upward" and "coarsening upward" are not generally applicable in carbonate rocks where in situ parti cle production results in highly vari able size, shape, and hydraulic den sity. Grain-sized parameters are thus unreliable if used indiscriminately. Basal-lag units may be comparable
Shelf
283
4
3
P L E I STOCE N E
2
Flgur� 1 4b-ldealized sequence. Units are: ( 1 ) Basal ca l c it i c lime mud of fresh wate r ponds (not represented ln co re); (2) Mangrove swamps that produce peat: (3) "Lake" sediments are shelly packstone lag with a few llthoclasts ; (4) Mud-bank sediments are pelleted mollusk wackestone: and (5) Island sediments are strcmatolitic pel let
wackestone
to mudstone. with peat and Perkins (1 977).
- lenses. From Enos
Figure 1 4a-Asymmetric transgressive
regressive sequence of Florida Bay.
Continuous plastic-impregnated core; top
is left .
from both grainy and muddy se quences. Specific examples range from conglomerates through floatstones to mud with a thin layer of coarser particles . Typical particles are intraclasts, lithoclasts from underlying sequences, and skeletal debris . Boring, breakage, encrusta tion, and metal-oxide staining of par ticles are common because accumula tion is slow . In fact , slow accumula-
tion because of low in situ production and/or periodic winnowing is prob ably more characteristic than surf zone reworking mentioned by James ( 1 977). Subtidal deposits may include any of the spectrum of lithofacies discuss ed above. Care must be taken to distinguish restricted deposits from deeper water, open-shelf deposits in this part of the sequence . Either may eventually be succeeded by intertidal sequences; the energy level of the shoaling succession may provide a clue to seaward barriers or restric tions in the lower part of the se quence. High-energy sands grading up into beach deposits or developing as an offshore shoal (oolite bar) suggest the lack of a significant barrier. A narrow shelly beach zone may,
however, develop in a restricted set ting such as the landward side of an enclosed lagoon . Beaches are developed locally in low energy set tings such as Biscayne Bay in south Florida, on islands in Florida Bay, along the west coast of Florida, and bordering the tidal flats of Andros Islands, Bahamas. Beach deposits may be overlain by supratidal deposits, ranging from muddy algal laminites or carbonaceous swamp deposits to eolian dunes, depending upon climate as well as energy level. A non-depositional weathering zone may cap the sequence. Characteristic features of such disconformities are root casts, soil structures, soil brec cias, laminated calcrete, caliche, etc. (Perkins, 1 977; Esteban and Klappa, 1 982).
284
Shelf
Figure 1 5-Shoaling-u pward sequence or "punct uated aggradational cyc le " wit h i n the Lowville Limestone of the Black River Group (M iddle Ordovician), l nghams M i l l s, New York. The base oi the cycle is at the color change just below the head of the hammer. The dark g ray lime wackstone is bioturbated and horizontally bu rrowed, contains gastropods, corals, and other marine fossils, and is apparently of shallow sub tidal origin, in contrast to the underlying, unfossil iferous, vertically burrowed l i me mudstone. The prominent tidal channel i n the center of the photo is overlain by vertically and burrowed mudc racked lime mudstone of tidal-flat origi n . Dark g ray subtidal wackestones at the top of the picture mark the base of the next cyc l e . Photograph courtesy of Pet e r Goodwin a n d E . A Anderson.
Fig u re 1 6-Shoaling-upward sequence within Manl i us limestone, H elderberg Group (Lower Devo nian), Perryville Quarry, New York . The cycle beg ins with an erosive base, 1 0 em above the lower end of the hammer handle. overlain by reworked stromatoporoid heads. Th is grades up i n to fossil-bearing lime wackestone and faintly laminated l i me mudstone. The brown dolomitic algal laminites form the top of the sequence. Photograph cou rtesy E. A. Anderson and Peter Goodwi n .
patch-reef or biohermal sequences of back-reef or atoll lagoons. Examples of shoaling-upward se quences within restricted en vironments have been described from the Cambro-Ordovician of the Cana dian Rocky Mountains (Aitken, 1 978) , the Ordovician Sto ny Moun tain Fo rm at ion of the western Williston Basin (Roehl, 1 967) , the Devonian Duperow Formation of the Beach deposits are probably less
common in restricted intertidal set tings than are muddy sediments characterized by lamination, fenestrae, oncolites, mud cracks and/or vertical burrows. The muddy
supratidal zone is characterized by
stromatolites , mud cracks, flat-pebble conglomerate, inter str at al dolomite, and evaporites . Other variants of shoaling-up sequences from a restricted subtidal setting include
Williston Basin (Wilson, 1 967), the Middle Carboniferous Windsor car bonates of the Maritime Provinces (Schenk , 1969), the Permian San An dres Formation of the Permian Basin (Chuber and Pusey, 1 967; Meissner, 1 972), the Upper Permian
Shelf
285
A
liT t ie re d maze
i r r e g u l a r l! o x w o r k
D Figure 1 7-Trace fossils commonly fou nd i n shallow-water carbonate en· vlron ments, Including semi-restricted set· tings. (A) Ophiomorpha, showing some of its morphologic range (from Frey et al, 1 978). (B) Thalasslnofdes. (C) Rizocorallium. (D) Chondrites. B to D rrom Hantzschel, 1 975.
Bellerophon Formation of the southern Alps (Bosellini and Hardie, 1 973), the Alpine Triassic (Assereto and Kendall, 1 97 1 ; Bosellini and Rossi, 1 974), the Jurassic (?) Arab/Darb Formation, Persian Gulf (Wood and Wolfe, 1 969), the Lower Cretaceous Sligo-Hosston Forma tions, south Texas (Bebout , 1 977), the Lower Cretaceous Edwards Group, central Texas (Fig. 1 1 ; Rose, 1 972), and the Holocene sediments of Abu Dhabi adjacent to the Persian Gulf (Fig. 12; McKenzie et al, 1980). Deepening upward or transgressive cycles characterize some restricted set-
tings. A well-known example is the Lofer cyclothems, lagoonal deposits formed behind the Dachstein barrier reef, Upper Triassic of the northern Alps (Fischer , 1 964) . In these cycles , red or green pelleted weathering residue (A, in Fig. 1 3) overlies and fills cavities within the deepest-water part of the cycle, the mega1odont (bivalve) limestone (C). The weather ing residue variously forms an argillaceous parting, fills solution and shrinkage cavities , or forms the matrix of a basal conglomerate. The succeeding "intertidal" deposits (B) contain crinkled and laminated
stromatolites, flat-pebble breccia, fenestrae, prismatic mud cracks, sheet cracks and a very restricted fauna of foraminifers, gastropods, and ostracods . The thickest member of the sequence is commonly an overly ing muddy to calcarenitic limestone (C) with a relatively diverse biota dominated by megalodont clams, red and green algae, oncoids, and en crusting foraminifera, interpreted as restricted subtidal deposits in a few meters of water (Fischer , 1 964) . The cycle is abruptly terminated with the appearance of a weathering residue. A sequence showing progressively
286
Shelf
Figure 1 a-. ·-B:.•rrows produced b'1' burrov.1ng f':hrimp, proba biy G::illii:ln:.l$S8. (see Fig. 4) . (A. right) 7-cm lmpragMtad core, S<>t..th Florida inn�r shelf margin (Fig .2a). (8, below) Unde rwater excavatiQn , Gre-at Ba,.. a ma Sank, sou thwest or Andros lsiMd (Fi9. 2b}. Water dep!h aho)ut 6 m . .
.
less restriction and probably a slight deepening upward has formed during the Holocene transgression of the south Florida inner shelf margin just seaward of the Florida Keys (Enos, 1 977) . A mollusk-fragment, lithoclastic lag deposit overlies a microkarstic weathering surface on Pleistocene coralline limestone . The lag deposit is overlain by muddy fine peloidal sediment (wackestone tex ture) containing a sparse foraminiferal fauna, few megafossils, and few burrows. This grades up into pelletal-skeletal "wackestone" (using the lithified equivalent) and ultimately packstone. Both are thoroughly bioturbated and contain diverse molluscan and foraminiferal faunas, codiacian green algae, and sea-grass roots. The grain-supported sediment represents the present depositional regime in the semi-restricted inner shelf m argin . An asymmetrical sequence compris ing a thin transgressive interval and regressive (progradational) interval records the Holocene transgression on the landward side of the Florida Keys in Florida Bay (Fig. 1 4) . Where mud banks have been colonized by mangroves to form islands, the
Shelf
287
Fig u re 1 9-Piastic cast of burrow ex· cavated by the shrimp Alpheus. Photograph courtesy of E. A. Shinn.
underlying sequence consists of (1) a basal calcitic mudstone of freshwater ponds, (2) peat from coastal mangrove swamps, (3) aragonitic skeletal packstone lag from the open bay and shallow basins between mud banks, (4) skeletal-pelletal wackestone of the mud banks with minor lenses of laminated pellet mudstone and skeletal packstone, and (5) stomatolitic pelletal wackestone with local peat lenses from supratidal islands (Enos and Perkins, 1 979). This idealized sequence is transgres sive through elements 1 to 3 and regressive through mud-bank and supratidal island phases , which are generally much thicker. The Black River (Middle Ordovi cian) and Helderberg (Lower Devo nian) Groups in the Appalachian Basin are well-documented transgres sive, upward-deepening sequences (Walker, 1972; Laporte, 1 969; Walker
and Laporte, 1 970). Both sequences measure a few tens of meters thick. Each begins with an intertidal supratidal facies mosaic that grades upward into shallow-water, subtidal muddy limestones with relatively diverse biota. The basal Manlius For mation of the Helderberg Group con sists of complexly interbedded pellet mudstones, wackestones, and dolomites with stromatolites, mud cracks, intraclasts, vertical burrows , and a restricted fauna including stromatoporoids, tentaculids, and gastropods (Laporte, 1 967). Low energy subtidal, i ntertidal, and supratidal depositional environments are represented. The succeeding Coeymans Formation is higher energy deposits including many encrinites that formed skeletal sand shoals and beaches (Anderson, 1 972). The overlying Kalkberg Formation is a cherty lime packstone and wackestone
with a diverse fauna including brachiopods, bryozoans, and trilobites, deposited on a low-energy, shallow-water "open shelf." The Kalkberg grades up into the more argillaceous, sponge-bearing New Scotland Formation (Laporte, 1 969) . Recent, more detailed work in pro gress suggests that these overall trans gressive sequences consist largely or entirely of small (a few meters) shoaling-upward intervals (Figs. 1 5 , 1 6 ; Anderson et a l , 1 978) . These in tervals or "punctuated aggradational cycles' ' represent progradational episodes superposed on progressive deepening of water and development o f less restricted environments. A similar progression of small shoaling upward cycles was noted by Bebout ( 1 977) within the generally transgres sive Hosston-Sligo sequence (Lower Cretaceous, south Texas). This re emphasizes the importance of the
288
Shelf
Figure 20-Fecal pellets of the burrow
ing s h r i mp Callianassa, filling a burrow i n a plastic-impregnated c o re H olocene, south Florida inner s h e l f m a rgin, 4 km .
east of Rodriquez Key (Fig. 2a). Pellets are about 1 mm in diameter.
burrows may indicate a lower popula tion of burrowers, or slower rates of burrowing relative to sedimentation, than do mottled or homogeneous sediments that can result from thorough bioturbation. A burrowing shoaling-upward cycles beginning in shallow subtidal settings, but does not infauna persists in oxygen-deficient alter the more regional significance of environments even where oxygen restriction has become too severe to overall transgressive units or negate support a shelly epifauna (dysaerobic the possible development of small facies, Byers, 1 977) . Corresponding scale transgressive cycles . The importance of sequence and cy observations have not been made for clicity of the various types described in other environmental restrictions such as salinity. placing shallow subtidal limestones in Restricted shelf, bay, and lagoonal their proper paleogeographic settings environments span parts of the can scarcely be overemphasized. shallow-water to intertidal Skolithus ichnofacies and shallow-shelf Cru Sedimentary Structures ziana i chn ofaci es o f Seilacher ( 1 967) . Burrows are the most common Shallow-water carbonates in general structures of restricted subtidal en are characterized by Ophiomorpha vironments . Incidence of burr owing (Permian to Recent; Skolithus facies) in restricted subtidal carbonates ranges from rare or lacking to and by Thalassinoides (Triassic to Ter thorough bioturbation. In fact , the tiary), Rhizocorallium (Cambrian to presence of well-preserved discrete Tertiary), and Condrites (Cambrian?,
Ordovician to Tertiary) , all of the Cruziana facies (Fig. 17). However ,
suites of ichnofossils specifically characteristic of restricted carbonate environments within this bathometric range have not been identified (Ken nedy, 1975) . Rhoads ( 1967) has observed that under environmental stress imposed by very shallow water and intermittent exposure, vertical burrows predominate over horizontal burrows and feeding traces . Vertical burrows are indeed more common in many of the environments of interest (Laporte, 1969). Small shoaling upward cycles in the Black River Group show repeated alternations from horizontal feeding traces in sub t i dal rocks to lo ng vertical burrows in presumed intertidal rocks (Fig. 15). Shinn ( 1 968b) described burrow ar chitecture in modern shallow-water carbonate environments. Shrimp, notably Callianassa and Alpheus, construct distinctive burrows (Figs.
Shelf
NW
ooc
2- 8 1 2
O DC 1 0 - 8 1 7
289
SE ODC 6-8 1 7
CDC 2 · 8 1 7
rm;�����! · =�;����
Figure 2 1 -Log response in cyclic car bonates from San Andres Formation (Guadalupian, Perm ian), Reeves fie ld, west Texas. Fou r numbered intervals near the base of each well are "backshelf cycles" from the landward part of the Reeves field. The top of cycle 4 co incides with the top of the main porosity zone. Logs are gamma ray (GR), neutron (N), and lateral log resistivity (ll) (from Chuber and Pusey,
1 969).
1 8, 1 9) that are most numerous in semi-restricted environments, although they persist to the shelf edge at depths less than 10 m (Shinn, 1968b). Callianassa burrows are of particular interest in that the complex spiral burrows with galleries and radiating dead-end tunnels (Fig. 4) are restricted to muddy environments . I n sandy sediment , Callianassa con structs simple straight or predominantly vertical tube networks w it h thick pelleted mud lining the walls. Identification is enhanced by Callianassa 's habit of stuffing dead end tunnels with one type of particle, for example coarse skeletal grains, grass blades, or fecal pellets. Callianassa 's perforated cylindrical
pellets (Fig. 20) further aid burrow recognition. Similar pellets are known at least as early as Triassic under the form name Favreina. The ichnofossil, Ophiomorpha, includes callianassid burrows but spans a wider range o f environments and probably was pro duced by other organisms as well (Frey et al, 1978) . The basis for definition of Ophiomorpha is a knob by burrow lining, not burrow pattern (Fig. 17a). Lamination is more typical of inter tidal and supratidal sediments than shallow subtidal sediments, but they are occasionally encountered in modern subtidal sediments and in an cient sediments interpreted as sub tidal. Laminites are common in organic-rich limestones where anoxic conditions have excluded burrowing organisms (Byers, 1 977), but most known examples are interpreted as basinal rather than restricted shelf in origin . The McKnight Formation (Cretaceous) o f south Texas (Rose, 1 972) and the Holzmaden beds of the Lower Jurassic o f southern Germany may represent restricted shelf en vironments. Macrobenthos are also
lacking o r severely reduced i n num bers of hypersaline lagoons. Here lamination or bedding is commonly distorted by dehydration shrinkage o f gypsum or by solution of evaporites. Lenses of laminated carbonate mud occur locally in less restricted settings such as Florida Bay. These may result from sedimentation from suspension in local depressions and from current reworking of low-density, aggregated sediment (pellets) as a traction carpet in slightly more exposed settings (Fig. 3; Enos and Perkins, 1 979) . The resulting deposits o f either of these features , one of negative relief and the other of positive relief, will ap pear as a small lens o f pelleted or clotted, laminated mudstone within coarser, bioturbated, pelletal-skeletal wackestone. Fenestral or "bird's-eye" structures - generally elongate synsedimentary pores larger t h an the surrounding grains and lacking obvious framework support - are likewise commonly associated with supratidal deposits but are also reported from restricted subtidal rocks. One example is from "unwinnowed shelf car-
290
Shelf
OAI
Figure 22-Gamma ray-neutron log response in cyclic Duperow Formation (Upper Devonian), southwestern North Dakota. Roman numerals indicate cyclic units defined on gamma ray-neutron logs, bounded by evaporites and silty dolomites (see text) (from Wilson, 1 967).
lO I
JACOBSON
PURE
� I
HO'fiZONTAl SCAL.£
noted , it is non-carbonate material punctuating the cycle which produces the response. The San Andres Forma tion (Guadalupian , Permian Basin, Texas-New Mexico) in the Reeves field, West Texas , exhibits marked cyclicity in both shelf-edge, high energy carbonates and equivalent lower energy "backshelf deposits" (Chuber and Pusey, 1 969) . The characteristic log response is a positive gamma-ray peak coupled with a negative "dense" deflection on the neutron log (Fig. 2 1 ) . This response is produced by a dark organic-rich shale interval at the base of the back-shelf cycle. In correlation of shelf carbonates within the Pearsall Formation (Lower Cretaceous, Texas; Loucks, 1 977) , R. G. Loucks (personal commun. , 1 979) noted that cycles or packages could be distinguished on mechanical logs only in the updip portions where tidal-flat deposits contain terrigenous clastic influxes. Well-developed cycles of normal marine carbonate, through restricted marine to evaporitic unfossilferous dolomite and anhydrite, characterize Log Response The common occurrence of the lower Duperow Formation (Fras nian, Upper Devonian) of the restricted shelf deposits in cycles that Williston Basin (Wilson, 1 967). The include adjacent facies suggests that dolomite-anhydrite lithology produces characteristic mechanical log a sharp gamma-ray peak normally responses should be recognizable. coupled with a dense response on the However , in the few published ex amples in which good log definition is neutron log (Fig. 22), although locally bonates" of the Upper Permian Tan sill Formation, Guadalupe Moun tains, New Mexico (Tyrrell, 1 969). The Tansill rocks are pellet, grapestone, and intraclast dolopackstones interpreted as com parable to the grass-covered inner shelf-margin sediments of south Florida (Tyrrell, 1 969) , although grains and textures are more analogous to those of the interior of the Great Bahama Banks. Shinn's statement ( 1 968a) in reference to distribution of fenestral pores in modern sediments that they " . . . are preserved in supratidal sediments . . . sometimes in intertidal sediments . . . and never in subtidal sediments. . . " is valid as a generalization with only a few exceptions. In summary, sedimentary structures are not the best guides in restricted subtidal environments, with the possi ble exception of a few distinctive bur row types. In fact, lack of hydro dynamic structures and high incidence of burrowing are typical . ·
GUNDERSON
AMERADA
... 2 KVAM
@
14 MILES APPRO.KIMATE!o.Y
the dolomite has more than l OOJo porosity (Wilson, 1 967). The anhydrite is very pure, but the dolomite commonly contains a few percent of detrital clay and silt which apparently produces the gamma-ray peak . Some cycles in each of these units exhibit tantalizing shapes sug gesting characteristic bell-shaped or funnel-shaped curves, but the shapes vary from cycle to cycle (Fig . 22). Economic Considerations
Porosity Potential
In general , the economic potential of the restricted facies in question is low . Carbonate mud has high initial porosity (60 to 750Jo ; Enos and Sawatsky, 1 98 1 ), but it is not preser vable through the neomorphic transi tion from the original elongate or ir regular, predominantly aragonite grains to the equant , loaf-shaped, calcite crystals typical of ancient lime mudstones (Folk, 1 965). Moreover, initial sediment permeabilities are relatively low, (0.6 to 1 00 md) in Florida Bay subtidal sediments (Enos and Sawatsky, 1 98 1 ) . Pelleted sediments do not have much higher permeabilities nor potential for porosity retention unless a significant number of the peloids are hard and therefore act as sand grains . Peloidal sands of the Great Bahama Bank have frame-supported porosities of 40
Shelf
[
291
A LT E R ED ZON E 50 feet
LIME MU DSTONE C HA L KY M A T R I X ( PE R M EA B I L I TY BAR R I E R ) BOU NDARY ZON E OF H I G H WAT E R · S AT U R AT IO N ( NO E F F E CT I V E WAT E R D R I V E ) S miles
to 500Jo and initial permeabilities o f I to 10 darcys, the upper limit ap proaching that of oolitic sand. Com mon association of restricted car bonates with terrigenous clays or evaporites further reduces reservoir potential by introducing barriers to fluid migration . Evaporites , moreover, commonly cause secondary plugging of initial pore space begin· ning with penecontemporaneous diagenesis . Thus most economically important restricted carbonates owe their productivity to diagenetic or tee· tonic processes. Common Diagenetic Patterns
Sea level forms the most important datum for early diagenesis, .which in many cases is the major determinant of ultimate economic potential . Of submarine diagenetic processes, cementation is the most relevant to reservoir potential and, in general, is more intense in shallow water, culminating in beachrock settings. The effects can be extensive plugging of pore space (Shinn, 1 969) or forma tion of a framework that can aid in retention of primary porosity (Purser , 1 969) . Subaerial processes can also be either beneficial (leaching) or detrimental (cementation) to porosity and permeability development . Carbonates from restricted en· vironments considered here are sub-
MICRODOLO M I T E & FRAGM E N TAL l i M E STO N E W I T H M I C RO DOLOM I T I C MAT R I X
Figure 23-Diagrammatic representation of stratig raphic trap in Midale trend (Mississ ippian) Saskatchewan. N umbers 1 to 3 are i ntervals in shoaling-upward sequences desc ribed in text (from l lling et al, 1 967).
( SO L U T ION G A S D R I V E )
tidal and are thus less readily effected by exposure than are shoreline facies and various kinds of buildups such as reefs and sand shoals. The cyclic nature of many shallow-water car bonates, however, commonly leads to subaerial exposure and consequent diagenesis at the end of each cycle. Therefore, the lower parts of the cy cle, the normal domain of restricted subtidal deposits, may share in the action through the development of a fresh water lens even if they are not actually exposed . Of course, the in itially low permeability would con tinue to retard diagenetic alteration. Wilson ( 1 975) characterized one of his three types of shelf cycles as "platform cycles with intense diagenesis." The results of exposure or fluctuation in sea level are most pronounced in the top portions of the cycle, typically supratidal or beach deposits, but may extend down to lower portions of the cycle . The ef fects include large scale shrinkage features such as sheet cracks (Fischer, 1 964) and desiccation polygons (Assereto and Kendall, 1 97 1 ) , soil and caliche horizons (Esteban and Klap pa, 1 982), internal sediment (Dunham, 1 969a), soil pipes, "vadose pisoids" (Dunham, 1 969b), teepee structures, Neptunian dikes (Fischer, 1 964), solution-collapse breccia, cave networks, and dolomitization. The
extent to which the net result shows enhanced or reduced porosity varies considerably from case to case . Dolomitization is particularly im portant to the reservoir properties of many carbonates from restricted en vironments . By most accounts, pro ductive dolomites are coarsely crystalline with connected inter crystalline porosity ("sucrosic" dolomites; Murray, 1 960) . Dolomitization is typically on a regional scale and is probably at tributable to seepage-refluxion (Adams and Rhodes, 1 960) or mixing zone (Badiozamani, 1 973) processes. In such cases, favorable porosity development in restricted carbonates stems from their location downdip from extremely restricted en vironments where evaporites are pro duced , or their location adjacent to areas of subaerial exposure where a fresh-water lens is developed . Of course, both processes may also be superimposed from a later deposi tional regime whose facies distribu tion may be very different . Fracturing is another method by which reservoir potential of otherwise unfavorable facies may be enhanced. Restricted subtidal facies are as likely to be affected by fracturing as the next facies, especially if they have been made more brittle through dolomitization .
292
Shelf
DOlOM ITE CAP - ROCK ANHY D R I TE
OR
MUDSTONE
AND SHALE
LOW PE R M EABILITY CARBON AT E
.� � Figu re 24-Seal formed by up-dip pinch out of algal-peloidal l imestone Into dolomitic-evaporative rocks, M idale trend (M ississippian) Saskatchewan {from l l l ing et .al, 1 967).
Petroleum Trap Potential
Trap potential in the simplest cases is controlled by relation either to depositional or structural highs. Restricted subtidal facies are neither depositional buildups nor the updip limits of carbonate units, thus their inherent potential is further reduced. Restricted facies are most likely to be favorable for trapping where adja cent , normally more favored facies are rendered unfavorable by secon dary processes. For example, land ward facies may become sealed so that h ydrocarbons migrating updip will be arrested in the restricted facies. This is the case in some major fields of the Midale trend (Mississip pian) of the northeastern Williston basin, Saskatchewan and North Dakota. Production from Restricted Shelf, Bay, and Lagoon Facies
( l ) Steelman-Lampman field, Midale trend (Mississippian) Williston Basin, Saskatchewan (Edie, 1 958; Ill ing et al, 1 967) - Carbonate rocks about 500 m thick were deposited during Early Mississippian in the Williston Basin. Maximum accumula tion was in the area near the present center of the basin and depositional strike parallels the present outline of the basin in many areas . Mississip pian strata are bevelled by a pre-
[
ANHYD R I T IC
O M I T E M U D S1'0N E � �� •u A ND SHALE SEAT - S E A L .
25
feet
A LG A L - P E L L E T l i M E S TO N E
Mesozoic unconformity and overlain by generally impermeable Triassic redbeds . In the Mission Canyon Formation , open-shelf limestones of the center of the basin grade into restricted car bonates and eventually evaporitic rocks at the northeast margin of the basin. The sequence is generally regressive or progradational. At the northeastern margin, progradation is accomplished by rhythmic shoaling upward sequences (Fig. 23) consisting of (1) peloidal and bioclastic packstones and wackestones, dolomitized toward the top of the in terval , overlain by (2) microdolomite with remnants of ostracods, oncoids, and gastropods, and (3) massive anhydrite and microdolomite with anhydrite stringers. Reservoir rock is developed in the dolomitized limestones of unit 1 with porosities of 10 to 2007o and permeabilities of 0.5 to 1 0 md, and in the dolomite of unit 2 with porosities of 20 to 35% and permeabilities of 5 to 50 md. Seals are formed by the overlying anhydrite (unit 3) and by updip transition into porous but impermeable " chalky" lime mudstone ( 15 to 25% porosity, 0.3 to 3 md permeability) which is further sealed by redbeds above the unconformity (Fig. 23) . Rocks of this configuration contain an estimated 600 million barrels of oil in place at
1 mile
Steelman-Lampman. Comparable fields are Weyburn ( 1 . 1 billion bar rels) and Midale (500 million barrels) . A different type of reservoir is form ed where the updip evaporite rhythms are entirely tight and form a seal above algal-peloidal limestones with porosities of 5 to 17% and permeabilities of 5 to 50 md (Fig. 24). This type of trap forms the Riva, Wiley, and Glenburn fields in North Dakota and subsidiary reservoirs at Steelman and Weyburn. (2) Reeves field (Permian), Yoakum County, Texas (Chuber and Pusey, 1 969) - Many moderate-sized oil reservoirs have been located in the cyclic San Andres facies (Guadalu pian, Permian) in West Texas and New Mexico (Chuber and Pusey, 1 969; Meissner, 1 972) . One example in which reservoir facies have been shown to be the more restricted part of the cycle is the Reeves field in Yoakum County, Texas (Chuber and Pusey, 1 969) . The field is located near the shelf edge at the north end of the Midland basin. Shelf-edge cycles are characterized by dolostone sequences 4 to 1 0 m thick with ( 1) a basal burrowed organic shale overlain by (2) dark-gray sparsely fossiliferous dolomudstone , (3) gray increasingly fossiliferous dolowackestone and (4) dolopackstone, and capped by (5) oolitic and fossiliferous
Shelf
5
4
3
2
dolograinstone (Figs. 2 1 , 25). Further onto the shelf, in part within the Reeves field , this cycle passes into one in which the shale, dolomudstone, and dolowackestone grade up into stromatolites with shrinkage cracks and birdseyes. Oil is produced primarily from moldic, fracture, and intercrystalline porosity in the dolowackestone; it is occluded from the dolograinstones by second ary plugging of the pores by anhydrite, apparently derived from adjacent sabkha cycles. Draping over a gentle pre-San Andres shelf edge has produced about 1 3 m of struc tural closure on the field. An oil col umn in excess of 30 m indicates that porosity pinchouts updip into the shelf cycles play an important role in reservoir development in this 20 million barrel field. Other examples in which major production is obtained from rocks probably dep o sited under restricted marine conditions include the Neoco mian limestones of the Lacq gas field, Aquitaine basin, France (Winnock and Pentalier, 1 970) and parts of the Asmari Limestone in southeastern Iran (Hull and Warman, 1 970). Many of the studies of shoaling-upward cycles mentioned above stem from in terest in petroleum potential of these rocks (Duperow Formation, Wilson, 1 967; San Andres, Meissner, 1 972; Arab-Darb, Wood and Wolfe, 1 969), although detailed field studies are not available . SELECTED REFERENCES
1 1--- 2 � m m ---f Fig u re 2o-shelf·e�e deposltloMI cyc le Fo rmat i on (Guadalupian, Permian), neeves f ield,
from San Andres
west Texas. Numbers oorrespor.d to inte r��ls doocribcd in tcK1 {from ChubGr a nd Pus ey, ' 969)
Adams, J. E . , and M. L. Rhodes , 1960, Dolomitization by seepage refluxion: AAPG Bull . , v. 44, p. 1 912-1920. Ahr, W. M . , 1 973, The carbonare ramp, an alternative to the shelf model : Trans., Gulf Coast Assoc. Geol. Soc . , v. 2 3 , p . 221-225. Aitken, J. D . , 1 978, Revised models for depositional grand cycles, Cambrian of the southern Rocky Mountains, Canada: Bull. Canadian Petroleum Geology, v. 26, p. 5 1 5-542. Anderson, E. J . , 1972, Sedimentary struc ture assemblages in transgressive and regressive calcarenites: Montreal, 24th Int. Geol. Cong . , Sec. 6, p. 369-378. Anderson, E. A. , P. W. Goodwin, and B. Cameron, 1978, Punctuated ag gredational cycles (P ACS) in Middle Ordovician and Lower Devonian se quences, in D. F. Merriam, ed . , 50th
293
Ann. Mtg. Guidebook, New York State Geol. Assoc. , p. 204-224. Assereto, R. L . , and C. G. St. C. Ken dall, 1 97 1 , Megapolygons in Ladinian limestones of Triassic of southern Alps; evidence of deformation by penecontemporaneous desiccation and cementation: Jour. Sed. Petrology, v. 41 , no . 3 , p. 715-723. Badiozamani, K., 1 973, The dorag dolo mitization model-application to the Middle Ordovician of Wisconsin: Jour. Sed. Petrology, v. 43, p. 965-984. Basan, P. B . , 1973, Aspects of sedimenta tion and development of a carbonate bank in the Barracuda Key, south Florida: Jour. Sed. Petrology, v. 43, no. I, p. 42-53. Bebout, D. G., 1977, Sligo and Hosston depositional patterns, subsurface of south Texas, in D. G. Bebout and R. G. Loucks, eds. , Cretaceous car bonates of Texas and Mexico; applica tions to subsurface exploration: Austin, Univ. Texas, Bureau Econ . Geol. , Rept. Invest. 89, p . 79-96. Bosellini, A . , and L. A. Hardie, 1973, Depositional theme of a marginal marine evaporite: Sedimentology, v. 20, p. 5-27. and D. Rossi, 1 974, Triassic car bonate buildups of the dolomites , northern Italy, in L. F. Laporte, ed. , Reefs i n time and space: SEPM Spec. Pub . 1 8 , p. 209-233. Byers, C. W . , 1 977, Biofacies patterns in euxinic basins; a general model, in H . E. Cook and Paul Enos, eds., Deep water carbonate environments: SEPM Spec. Pub. 25, p. 5- 1 8 . Carrillo, B. J . , 1 97 1 , La plataforma Valles-San Luis Potosi: Asoc. Mexicana Geologos Petroleros Bol . , v. 23, p. . 1-101 . Chuber, S. , and W. C. Pusey, 1967, Cy clic San Andres facies and their rela tionship to diagenesis, porosity, and permeability in the Reeves field, Yoakum County, Texas, in J. C. Elam and S. Chuber, eds. , Cyclic sedimenta tion in the Permian Basin: Midland, West Texas Geol. Soc . , p. 1 36- 1 5 1 . Cloud, P . E . , Jr . , and V . E . Barnes, 1956, Early Ordovician sea in central Texas, in H. S. Ladd, ed. , Treatise on marine ecology and paleoecology: Geol. Soc. America Mem. 67, v. 2, p. 163-2 14. Conrad , M . A . , 1 977, The Lower Creta ceous calcareous algae in the area sur rounding Geneva (Switzerland); biostratigraphy and depositional en vironments, in E. Fluegel, ed. , Fossil algae: Berlin , Springer-Verlag Pub . , p. 295-300. Daetwyler, C. C . , and A. L. Kidwell, 1 959, The Gulf of Batabano, a modern carbonate basin: Proc . , 5th World __
294
Shelf
Petroleum Cong . , Geology and Geophysics, Sec. 1 , p. 1-22. Davies, G. R., 1 970, Carbonate bank sedimentation, eastern Shark Bay, Western Australia, in B. W. Logan et a!, eds. , Carbonate sedimentation and environments, Shark Bay, Western Australia: AAPG Mem. 1 3 , p. 85- 1 68. Dunham, R. J . , 1 969a, Early vadose silt in the Townsend mound (reef), New Mexico , in G. M. Friedman, ed . , Depositional environments in carbonate rocks: SEPM Spec. Pub. 14, p . 1 39- 1 81 . 1969b, Vadose pisolite in the Capitan Reef (Permian), New Mexico and Texas, in G. M. Friedman, ed. , Depositional environments i n carbonate rocks: SEPM Spec. Pub . 14, p . 1 82- 1 9 1 . Ebanks, W . J . , Jr., and J . N . Bubb, 1 975 Holocene carbonate sedimentation, Matecumbe Keys tidal bank, south Florida: Jour. Sed. Petrology, v. 45, no . 2, p . 422-439. Edie, R . W . , 1 958, Mississippian sedi mentation and oil fields in southeastern Saskatchewan : AAPG Bull. , v. 42, p . 94- 126. Enos, Paul, 1974a, Reefs, platforms, and basins in middle Cretaceous in nor theast Mexico: AAPG Bull. , v. 58, p . __
800-809.
1 974b, Surface sediment facies map of the Florida-Bahamas Plateau: Geol. Soc. America, Map Series, MC-5, 4 p. 1977, Holocene sediment accumu lations of the south Florida shelf margin, in P. Enos and R. D. Perkins, eds . , Quaternary sedimentation in south Florida: Geol. Soc. America Mem. 147, pt. I, p . 1- 1 30. and R. D. Perkins, 1979, Evolu tion of Florida Bay from island stratigraphy: Geol. Soc. America Bull . , Part I , v. 90, p . 59-83. and L. H. Sawatsky, 1 98 1 , Pore space in Holocene carbonate sediments: Jour. Sed. Petrology, v. 5 1 ; no. 3, p . 96 1 -985. Esteban, M . , and C. Klappa, 1 982, Sub aerial exposure surfaces, in P. A. Scholle, D. Bebout, and C . Moore, eds . , Carbonate depositional en vironments: AAPG Mem. 33, this volume . Fischer, A. G . , 1 964, The Lofer cyclo thems of the Alpine Triassic, in D. F. Merriam, ed., Symposium on cyclic sedimentation: Kansas Geol. Survey Bull. 169, v. I , p. 107- 149. Fluegel, E., 1977, Environmental models for Upper Paleozoic benthic calcareous algal communities, in E. Flugel, ed. , Fossil algae: Berlin, Springer-Verlag Pub. , p. 3 1 4-343. Folk, R . L . , 1 965, Some aspects of ___
___
__
__
recrystallization in ancient limestones ,
in L. C. Pray and R. C. Murray, eds . ,
Dolomitization and limestone diagenesis: SEPM Spec. Pub. 1 3 , p. 14-48. Frey, R. W . , J. D. Howard , and W. A. Pryor, 1 978, Ophiomorpha; its mor phologic, taxonomic, and environmen tal significance: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 23, p . 1 99-229. Ginsburg, R. N . , 1956, Environmental re lationships of grain size and constituent particles in some south Florida car bonate sediments: AAPG Bull . , v. 40, p. 2384-2427. Hantzschel, W . , 1 975, Trace fossils and problematica, in C. Teichert , ed. , Treatise o n invertebrate paleontology: Geol. Soc. America, Univ. Kansas Press, Part W, suppl. I , 2nd ed . , 269 p. Heckel, P. H . , 1973, Nature, origin , and significance of the Tully Limestone; an anomalous unit in the Catskill delta, Devonian of New York: Geol . Soc. America Spec. Paper 138, 244 p. and J . F. Baesemann, 1975, En vironmental interpretation of conodont distribution in Upper Pennsylvanian (Missourian) megacyclothems in eastern Kansas: AAPG Bull. , v . 59, p. 486-509. Hine, A. C . , 1977, Lily Bank, Bahamas; history of an active oolite sand shoal: Jour. Sed. Petrology, v. 47, no. 4, p . ___
1554- 1 58 1 .
Houbolt, J . J . H . C . , 1 957, Surface sed iments of the Persian Gulf near the Qatar Peninsula: The Hague, Mouton, l l3 p . Hull, C . E . , and H . R . Warman, 1 970, Asmari oil fields in Iran, in M. T. Halbouty, ed., Geology of giant petroleum fields: AAPG Mem. 1 4, p . 428-437. Illing, L. V . , G. V. Wood, and J. G. C. M . Fuller, 1967, Reservoir rocks and stratigraphic traps in non reef carbonates: 7th World Petroleum Cong . , v. 2, p. 487-499. Irwin, M. L . , 1 965 , General theory of epeiric clear water sedimentation: AAPG Bull. , v. 49, p. 445-459. James, N. P . , 1 977, Facies models 8; shallowing-upward sequences in car bonates: Geoscience Canada, v. 4, no. 3 , p . 1 26- 1 36. Jindrich, V . , 1 969, Recent carbonate sedimentation by tidal channels in the Lower Florida Keys: Jour. Sed. Petrology, v. 39, p . 53 1-553. Kennedy, W . J., 1 975, Trace fossils in carbonate rocks, in R. W. Frey, ed. , The study o f trace fossils: New York, Springer-Verlag Pub . , p. 377-398. Laporte, L. F., 1 967, Carbonate deposi tion near mean sea-level and resultant facies mosaic; Manlius Formation
(Lower Devonian) of New York State: AAPG Bull. , v. 5 1 , p. 73- 1 0 1 . 1969, Recognition o f a transgres sive carbonate sequence within an epeiric sea; Helderberg Group (Lower Devonian) of New York State, in G . M. Friedman, ed. , Depositional en vironments in carbonate rocks: SEPM Spec . Pub. 14, p. 98- 1 19. Logan, B. W., et a!, 1 970, Carbonate sedimentation and environments, Shark Bay, Western Australia: AAPG Mem. 1 3 , 223 p. Loucks, R. G . , 1977, Porosity develop ment and distribution in shoal-water carbonate complexes-subsurface Pear sall Formation (Lower Cretaceous) south Texas, in D. G. Bebout and R . G. Loucks, eds . , Cretaceous carbonates of Texas and Mexico: Austin, Univ. Texas , Bureau Econ. Geol. , Rept . In vest . no . 89, p. 97- 126. McKenzie, J. A., K. J . Hsu, and J. F. Schneider, 1980, Movement of subsur face waters under the sabkha, Abu Dhabi , UAE, and its relation to evaporative dolomite genesis, in D. H . Zenger , J. B . Dunham, and R. L . Ethington, eds., Concepts and models of dolomitization: SEPM Spec. Pub. 28, p. 1 1 -30. Meissner, F. F., 1 972, Cyclic sedimenta tion in Middle Permian strata of the Permian Basin, West Texas and New Mexico, in J. C. Elam and S. Chuber, eds . , Cyclic sedimentation in the Per mian Basin: Midland, West Texas Geol. Soc . , 2nd ed. , p. 203-232. Moore, R. C . , ed. , Treatise on in vertebrate paleontology: Geol. Soc. America, Univ. Kansas Press, 24 parts. Murray, R . C . , 1 960, Origin of porosity in carbonate rocks: Jour. Sed. Petrology, v. 30, p. 59-84. Neumann, A. C . , 1965, Processes of Recent carbonate sedimentation in Har rington Sound, Bermuda: Bull. Marine Sci. Gulf Caribbean, v. 1 5 , p. 987- 1 035. Newell, N . D., et a!, 1 953, The Permian reef complex of the Guadalupe Mou tains region, Texas and New Mexico : San Francisco, Freeman and Co. Pub . , 236 p . Perkins, B . F., 1974, Paleoecology o f a rudist reef complex in the Comanche Cretaceous Glen Rose Limestone of central Texas: Geoscience and Man, v. 8, p. 1 3 1 - 173. Perkins, R . D . , 1977, Pleistocene depo sitional framework of south Florida, in Paul Enos and R . D. Perkins, eds. , Quaternary sedimentation i n south Florida: Geol . Soc. America Mem. 1 47, p. 1 3 1 - 198. Price, W. A., 1 967, Development of the basin-in-basin honeycomb of Florida Bay and the northeastern Cuba lagoon: Trans., Gulf Coast Assoc. Geol. Soc . , ___
Shelf
v. 17, p. 368-399. Purdy, E. G . , 1974a, Reef configuration; cause and effect, in L. F. Laporte, ed . , Reefs in time and space: SEPM Spec. Pub. 1 8 , p. 9-76. 1974b, Karst determined facies patterns in British Honduras; Holocene carbonate sedimentation model : AAPG Bull. , v. 58, p. 825-855. Purser, B. H . , 1969, Syn-sedimentary marine lithification of Middle Jurassic limestones in the Paris Basin: Sedimen tology, v. 1 2 , p. 205-230. __ ed . , 1 973, The Persian Gulf; Holocene carbonate sedimentation and diagenesis in a shallow epicontinental sea: Berlin, Springer- Verlag Pub . , 47 1 p. Read , J . F . , and G. A. Grover, Jr. , 1977, Scalloped and planar erosion surfaces, Middle Ordovician limestones, Virginia; analogues of Holocene exposed karst or tidal rock platforms: Jour. Sed. Petrology, v. 47, p. 956-972. Rhoads, D. C . , 1 967, Biogenic reworking of intertidal and subtidal sediments in ___
Barnstable Harbor and Buzzards Bay,
Massachusetts: Jour. Geology, v. 75 , p . 46 1-476. Roehl, P. 0 . , 1 967, Stony Mountain (Or dovician) and Interlake (Silurian) facies analogs of Recent low-energy marine and subaerial carbonates, Bahamas: AAPG Bull . , v. 5 1 , p. 1 979-2032. Rose, P . R . , 1972, Edwards Group, sur face and subsurface, cent ral Texas : Austin, Univ. Texas, Bureau Econ. Geol . , Rept . I nvest . 74, 1 98 p. Schenk, P . E., 1969, Carbonate-sulfate redbed facies and cyclic sedimentation of the Windsorian Stage (Middle Car boniferous) Maritime Provinces: Can. Jour. Earth Sci . , v. 6, p. 1037- 1066. Seilacher, A . , 1967, Bathymetry of trace fossils: Marine Geol . , v. 5, p . 41 3-428. Shaw, A. B . , 1 964, Time in stratigraphy: New York, McGraw-Hill Pub . , 353 p . Shinn , E. A . , 1 968a, Practical significance of b irdseye structures in carbonate ' rocks: Jour. Sed . Petrology, v. 38, no. I , p. 2 1 5-223.
, 1 968b, Burrowing in recent lime sediment of Florida and the Bahamas : J o ur. P aleonto lo gy , v. 42, no. 4, p. 879-894. , 1 969, Submarine lithification of Holocene carbonate sediments in the Persian G ul f: Sed iment ology, v. 1 2, p . 109-144. 1 982, Ancient carbonate tidal fl ats, in P. A. Scholle, D. Bebout, and C. H. Moore, eds . , Carbonate deposi tional environments: AAPG Mem. 33, this volume. R. M. Lloyd, and R. N. Gins burg, 1 969, Anatomy of a modern car bonat e tidal-flat, Andros Island, Bahamas: Jo ur . Sed. Petrology, v. 39, no. 3, p. 1 202- 1 228. Smith, C . L., 1940, The Great Bahama Bank: Jour. Marine Research, v. 3, p . 1 -3 1 , 1 47- 1 89. Spackman, W., D. W . Scholl, and W. H. Taft, 1964, Environments of coal for mation in south Florida: Miami Beach, Geol . Soc. America Guidebook Field Trip 5, 67 p . Swinchatt, J . P . , 1 965 , Significance o f constituent composition , texture, and skeletal breakdown in some Recent car bonate sediments: Jour. Sed. Petrology, v. 35, no. 1 , p. 7 1 -90. Tsien, H. H . , and E. Dricot , 1977, De vonian calcareous algae from the Di nant and Namur basins, Belgium, in E. Fluegel , ed . , Fossil algae: Berlin, Springer-Verlag Pub . , p. 344-350. Turmel , R. J . , and R. G. Swanson, 1 976, The development of Rodriguez bank, a Holocene mudbank in the Florida reef tract : Jour. Sed. Petrology, v. 46, no. 3, p. 497-5 1 8 . Tyrrell, W. W . , Jr . , 1969, Criteria useful in interpreting environments of unlike but time-equivalent carbonate units ___
___
___ ,
__
(Tansili-Capitan-Lamar) , Capitan Reef
Complex, West Texas and New Mexico,
in G. M. Friedman, ed . , Depositional
environments in carbonate rocks: SEPM Spec. Pub. 14, p. 80-97. Viniegra 0 . , F., 1 97 1 , Age and evolution of salt basins of southeastern Mexico:
295
AAPG Bull . , v. 55, p. 478-494. Walker, K. R., 1972, Community ecology of the Middle Ordovician Black River Group of the New York State: Geol. Soc. America Bull . , v. 83, no. 8, p . 2499-2524. and L. F. Laporte, 1 970, Con gruent fossil communities from Ordovi cian and Devonian fossil communities of New Yor k : Jour. Paleontology, v. 44, p. 928-944. Walther, J . , 1 894, Einleitung in die Geol ogie als historische Wissenschaft: Jena, Fischer Verlag, v. 3, 1 055 p . Wanless, H. R . , 1969, Sediments o f Bis cayne Bay-di st ribut i on and depositional history: Univ. Miami (Fla.), lnst. Marine Atmos. Sci . , Tech . Rept. 69-2, 260 p. Wantland, K. F., and W. C . Pusey, III , eds . , 1975, Belize shelf-carbonate sediments, clastic sediments, and ecology: AAPG Stud. in Geology No . 2, 599 p . Wilson, J . L . , 1967, Carbonate-evaporite cycles in lower Duperow Formation of Williston basin : Canadian Petroleum Geol . Bull., v. I S , p. 230-3 1 2 . 1975, Carbonate facies i n geologic history: New York, Springer-Verlag Pub . , 471 p. Winland, H . D., and R . K. Matthews, 1974, Origin and significance o f grapestone, Bahama Islands: Jour. Sed . Petrology, v. 44, n o . 3, p . 921 -927. Wi nn o ck, E . , and Y . Pentalier, 1 970, Lacq gas field, France, in M. T. Halb outy ed. , Geology of giant petroleum fields: AAPG Mem. 14, p . 370-387. Wood, G. V., and M. G. Wolfe, 1 969, Sabkha cycles in the Arab-Darb Forma tion off the Trucia1 Coast of Arabia: Sedimentology, v. 12, p. 1 65- 1 9 1 . Zenger, D. H . , 197 1 , Uppermost Clinton (Middle Silurian) stratigraphy and petrology east-central New York: N.Y. State Mus. and Sci. Service Bull. 417, 58 p. ___
___
,
298
Middle Shelf
BAS I N
SLOPE
SHELF
I
n the constant struggle of tectonic uplift versus subsi dence and sedimentation, a number of recognizable sed imentary patterns may become estab lished during the various stages of basin-filling . Carbonate shelf facies form one of these patterns. They develop in shallow subtropical to tropical seas where little or no ter rigenous material is introduced . The rate of sedimentation is rapid and , though sporadic, can keep pace with tectonic subsidence. For this reason the bulk of carbonate sediment which builds out from cratonal positive areas and fills adjacent basins is deposited in the shallow shelf en vironment . The term shelf has always been used somewhat loosely by geologists; here it refers to areas of extensive shallow-water environments bordered on their shoreward side by nearshore or continental sediments and offshore by slope and basinal sediments. Sub division of the idealized shelf spec trum of carbonate facies, first outlin ed by Irwin ( 1 965) , was expanded by Wilson ( 1 970, 1 974) who defined nine basic facies belts. Shelf facies may in clude six of these belts (Nos. 4 through 9) as shown in Figure 1 . In general , the term shelf facies is used here for middle or open shelf sediments. The deposits are con tinuous, widespread sheets formed predominantly from carbonate sediments produced in shallow-water environments. The distinctive sediments of the in ner shelf (restricted marine} , outer shelf, and slope are discussed in other chapters in this volume. Deposits of the middle shelf, mostly of normal marine salinity, are the subject of this chapter. They have the following general characteristics: ( 1 ) sea water mostly of normal salinity; (2) depths varying from a few tens of meters
tON
7 f'
A
C
E
S
B
E
L
T S
Figure 1 a-Profi le of shelf facies show major su bdivisions of s hel f and facies subdivision of middle shelf such as patch reefs (cora l and red algal i ng
boundstones), m ud mounds (algal plate wackestones to mudstones), grainstone shoals (oolite, pellet, or bioclastic grai nstones) and depressions filled with
carbonate mud.
sequences of limestone or dolomite. (4) Sedimentary structures such as extensive bioturbation , burrows, nodular bedding, and flaser bedding are typical. SETTING
(actually local developments of emergent bars or islands are only 0 to 2 m above sea level) to one or two hundred meters; (3) temperatures ranging from 10 to 30°C (subtropical to tropical) ; (4) generally well oxy genated water; and (5) bottom condi tions commonly below normal wave base, but above storm wave base. There are several diagnostic criteria for the recognition of shelf facies: ( 1 ) The biota include various stenohaline forms (that is, those with a low tolerance to salinity variations from normal sea water), commonly referred to as "normal marine faunas . " (2) Carbonate textures are general ly muddy, with packstones and wackestones dominating shelf en vironments; but patch reefs and sand shoals also occur, producing local ac cumulations of boundstones and skeletal grainstones. (3) Bedding is of variable thickness with lens- or wedge-shapes common. Thin shale beds may interrupt shelf
Tectonically, carbonate shelves are best developed on the margins of cratonic blocks, in intracratonic basins, across tops of major offshore banks, and on localized positive elements on wide shelves. The thickness of shelf deposits is a func tion of the stability of the shelf relative to sedimentation rates . Car bonate depositional rates are high, and subsidence is rarely rapid enough to inhibit the system's ability to pro duce sediment. In most cases , sedimentation far exceeds subsidence . Some of the thickest carbonate sec tions in the world consist of pro grading shelf facies deposited on con tinental margins. In the Triassic Dolomites of northern Italy and in the Cretaceous of central Mexico , 2,000 to 3 , 000 m of carbonate shelf deposits were produced over large offshore banks during the time of one or two stages. The particular depositional setting of shelf facies is influenced by tec tonic controls, the outer shelf profile,
Middle Shelf
299
� L...amtnated � Bunowed
2S,
4-
CrOM·t>eclded
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• I
Sealod .,cture.s f'rlled
vugs
� Geoi>etal -- Stvlolrtes ® Iron stai'led � Wa.� or nodular
0
Massive corals 'f Stanching corals CD Molfuscs {unditf) ll'l �ollusc ftiot!IIIMlh� - Pelecypods
._. Oysters
10 Gastropods
P Monoplounds .9 Capnnids
Jl H� (u..,lff)
� ToucasidS � Radlollllds V Caprolinicls
.-. * .c ·ft 'l7
Brachoopods Crinoid .EChinoid BI)'OZOO (f<:nC31rolo)
BI)'OZDa lr"""'"") 4i' Tnlobte
A Forams (unditf', 9> Plt�.nklunics l) LS.rge benlhonfcs ......, Tubular rorarns .,. Fosulinids A. Enr.m"tlng toram.s A Spiculefl o Oslracods
Figure 1 b-List Figure 1 a.
of symbols used in
of shelf edge barriers effective enough of the vast expanse of shallow shelf to restrict the exchange of water waters result in faunal restriction. across middle and inner shelf en STRATIGRAPHY AND FACIES vironments is common throughout geologic history. Most examples of severe salinity restriction are from in Shelves and their margins follow ner shelf environments, where typical two main patterns which we term barriers include mudbanks, islands, ramp and drop-off models. mangroves, swamps, and broad, slug Shelves with profiles of the ramp gish shelf lagoons. Depending on model have broad, gradational facies climate, the type of salinity restriction belts. These sediments are widespread and were deposited in layercake may be hypersaline ( > 36 ppm NaCI - usually associated with arid fashion as broad sheets or wedges. regions) or hyposaline ( <36 Isopach maps of these sediments should reflect the relatively uniform ppm - usually associated with tropical areas with considerable fresh sedimentation rates. The width of water runoff). these shelves is highly variable and "Restricted" Conditions ranges from ten to several hundred Faunal restriction - Geographic Geographic restriction - This in and salinity restrictions result in little kilometers. The width of ramp-model vokes the existence of barriers to facies belts depends on the angle of or no biotic content in the sediment form a lagoon; the barriers may be inclination of the ramp (the sloping or in simple fossil assemblages with shelf-edge reefs or oolite bars, coastal seafloor); gently inclined ramps low diversity, high-number popula irregularities of the inner shelf, dense generally occur on wide shelves and tions. This is caused by the ability of reef clusters of the middle shelf, or one or two well-adapted organisms to yield irregular facies belts. simply an excessive shelf width not Shelves with profiles of the drop multiply in a stress environment with allowing adequate water circulation. essentially no competition from other off model have well-developed linear This last type of barrier results when facies belts, trending parallel to the organisms. With regard to complete frictional forces completely consume faunal restriction, there are several shelf edge. Isopach maps indicate that marine energies trying to move large . carbonate facies of the drop-off examples in the geologic record of volumes of water across broad, shallow shelves . These energy losses basically non-bioclastic shelf sedimen model are thickest at the outer shelf, especially at the prograding shelf across shelf lagoons most strongly af- . tation. The modern-day Great Bahama Bank is a good example, feet sedimentation in inner shelf en margin . Thinner (and more compac table) beds o f the basin occur on one with its pelletal grainstone/packstone vironments. Shaw (1964) discussed side of the thick outer shelf facies, facies broadly extending across most this phenomenon in depth and of the shelf environment. Hypersaline and thinner beds of the inner and described examples mainly from in conditions, lack of nutrients, slight middle shelf facies occur on the other tracratonic basins. stagnation, and elevated temperatures side. The internal geometry of dropSalinity restriction - The existence and to some extent, by the age of geologic time represented . Thus, the type of carbonate facies formed at the shelf edge depends on the character of reef-building organisms, and this in turn controls circulation over the entire shelf. The width of the shelf and the effectiveness of barriers at the shelf edge directly influence the composition of shelf facies. On especially wide, shallow shelves or on shelves with a very well-developed barrier at the shelf edge, restricted shelf facies are deposited (Enos, 1 982).
300
Middle Shelf
Figure 3-Va riable and i r reg u la r bed thickness typ i c a l of shelf limestones. Al t ern ati on l ime-wackestone and dark shale beds of the shallow Koessen b asi n . . below the Late Triassic Steinplatte Reef, . F1gure �-�pward shoaling �ed1menta_ry northern Limestone Alps, Waidring, cycles 1 n m t ddle Pennsy lvan i a n st rat a 1 n Au st r i a . D eposi ted in 1 00 m of water . Sterra de las Palomas, n or ther nmost Chi h u ahua. Recessive bands c on s i st of slig htly argillaceous dark limestones with some marine fossi ls, lower parts of cl iffs consist of fusulinid l imes to nes and normal marine units comprise the ear massive top of each ledge has a ly part of each cycle and are replaced g rainstone to packstone texture. Very upward by restricted marine facies, top of ledge commonly d isplays a and, in some examples, ultimately by lithoclastic sedimentary breccia ove r l a i n evaporite beds (Figs. 2, 27). Both directly by a rgillaceous strata of next cy· uniform sheets and upward shoaling cle (see also Fig. 27).
off model carbonates usually reflects deposition under classic regressive conditions - somewhat as a large scale foreset/topset type o f basin filling . Several identifiable stratigraphic patterns occur across shelf areas. The first is a widespread sheet limestone with more or less uniform normal marine faunas and considerable con sistency in sedimentary facies over vast areas . The Wolfcampian Hueco Limestone of west Texas and southern New Mexico and Trenton Galena-Red River-Big Horn-Viola Montoya unit of Middle to Late Or dovician age are good examples. A second stratigraphic pattern is typified by upward shoaling sedimen tary cycles (Wilson, 1 975) wherein
sequences may change facies to more restricted marine facies over slight paleo-highs on the shelf and in shallow mud-filled depressions on the shelf where dark laminated , argillaceous limestones accumulate. Usually narrow linear facies belts oc cur close to barriers at the shelf margin, and wide, irregular patterns occur on shallow shelves . Stratigraphic patterns of carbonate sedimentation in middle shelf en vironments may be strikingly inter rupted by shale units representing periodic incursions of clay. These shales are commonly gray-green or brown across the high parts of the shelves; they are noticeably darker in the basins. Shallow intracratonic basins within broad shelves may be filled in two ways (Wilson, 1975). Slight, gradual, but continuous subsidence results in stratigraphic units passing into the
basin without much facies change, but thickening two or three times basinward. Cyclic-reciprocal sedimen tation forms another type of basin filling pattern. During high sea level stands , carbonate fringes or shallow platforms develop around the basin, its central part being generally starved of sediment . The very thin basinal section equivalent to the carbonate fringe is submerged in somewhat deeper water. At lower sea level stands, this basin becomes filled with terrigenous clastics which by-pass the shelves, or thick basinal evaporites may form if proper climate and restriction prevail. DESCRIPTION OF SHELF FACIES Regional Facies
. Wackestone textures dominate shelf facies; mudstones and packstones are also very common. Shelf carbonates range in color from light to dark gray with lighter shades being typical since most shelves are well oxygenated. The periodic incursions of clay result in discrete stratified shales and are prob ably due to mild distant tectonic uplift and erosion or climate change on a distant land mass. These shales are commonly calcareous (m arls) and may be extremely fossiliferous. The basic association of tabular shelf limestones with gray-green or brown
Middle Shelf
301
Figure 4-Even rhythmic bedding with argil laceous siliceous partings in pelagic l i mestones of M iddle Cretaceous Cuesta del Cura Formation. Canyon de los Chor ros, 30 km south of Saltillo, Coahuila, Mexico. Contrast this bedding with i r regular bed thickness of open-m arine shallow shelf facies.
shales is most typical of the shelf environment. The biota of middle shelf car bonates is usually diverse and in dicates widespread normal-marine salinities. A lack of fossils or severely limited fossil diversity is caused by a stress environment resulting from an effective shelf barrier of some sort restricting water circulation and salinity across the shelf. Microfacies
In the vast blanket of skeletal/pelletal wackestones that oc curs across most carbonate shelves, three special microfacies occur. These textures are illustrated in photomicrographs of ancient ex amples of shelf limestones . Mudstones and skeletal wacke stones - These fine-grained car bonates are deposited in localized low places on the shelf to form mud-filled depressions. Hawk Channel behind the Florida Reef Tract is a good Holocene example of a linear depres sion being filled with muddy skeletal carbonates . Similar low areas with muddy carbonates are known across the Great Pearl Bank of the Persian Gulf. Grainstones and packstones Shallow shoals are common in shelf settings. The main particle types are ooids, peloids, or bioclasts. Cross bedding and shallow-water fossils reflect the relatively high energy con-
ditions. Emergent shoals may develop local beach and supratidal facies. Thicknesses of these middle shelf facies are much less than their facies counterparts on the outer shelf. Boundstones and flanking skeletal grainstones/packstones - Several types of localized organic buildups occur across carbonate shelf en vironments, becoming sites of deposi tion for everything from a rigid reef framework, to piles of reef detritus, to mud mounds with algal plates or mollusks. These buildups may be categorized by their geometry and their position on the shelf. Patch reefs have an irregular, but basically circular pattern in plan view and are found only in shelf environments. Details of their facies have been studied in Bermuda, the Florida Reef Tract , and on the shelf behind the Belize barrier reef (Rhomboid Shoals) . Patch reefs are generally very steep-sided and rise sharply from a flat lagoon floor. Their height or ver tical relief is naturally limited by shallow water depths. In contrast, small reefs in deeper water on the outer shelf and slope are referred to as knoll reefs . They develop best on gently dipping slope sediments. They generally are low relief features and contrast with pin nacle reefs which in basinal en vironments form very steep-sided structures with considerable relief of up to hundreds of meters.
Fig u re 5-Ciose up of Po rtlandian ( Late J u rassic) strata in southeast Paris Basin at Ville A rvo n a l near Chaumont. Wavy nodu lar bedding caused by differential compaction of varying carbonate and argi l laceous content of partings between strata.
Patch reefs and lime mud mounds within shelves tend to be more ir regular both in shape and trend than such structures at shelf margins. At the shelf edge, they are commonly elongate or loaf-shaped, and directed either parallel to the shelf margin or perpendicular to it. In middle shelf environments, buildups may be con trolled by underlying topography or previous reef positions; they may also occur as piles of sediment built out from fault scarps or growing positive areas . In such cases, thicker sediments commonly accumulate on the downthrown sides of faults or on the flanks of paleohighs. Paleoecology
Fossils found in deposits on shelves with open circulation and normal marine salinities are stenohaline. This
302
Middle Shelf
Figure 6-Arg illaceous thi n-bedded limestone and marl of open shelf fossiliferous wackestone at base of Callovian (Ju rassic) upward shoaling cy cle, Etrochey Formation, Cu re Valley, southeast Paris Basin . Note nodular bed d i ng and irregular thickness of strata.
varied , and commonly thicker-walled. is true for most carbonate shelves These include fusulinids and en throughout geologic time, except for restricted environments of some inner dothyrids in the Paleozoic, and shelves where a very limited biota ex miliolids and alveolinids in younger ists (Enos, 1 982) . strata. Encrusting forms such as Normal marine salinity is indicated Homotrema and cornuspirids/ calcitornellids and the dubious by the following fossil groups: foraminifer Tubiphytes require hard brachiopods, echinoids, crinoids, nautiloids , and ammonites, as well as substrates on which to attach . This limits their distribution on the shelf by many individual genera of other fossil groups . The presence of marine to patch reefs , rocky crags, or possibly submarine cemented crusts. calcareous algae indicates water dep Mollusks are also sensitive to con ths within the photic zone which ex tends. down to about 1 00 m . Common ditions of salinity and marine energy across the shelf. Basically, the thick algae found in shelf environments in shelled, robust, and commonly larger clude phylloid (platy) algae of the Late Paleozoic and codiacean algae in mollusks live in high-energy shoal en general . Red algae also generally vironments. On the other hand, smaller thinner-shelled forms live in thrive in water of normal marine salinity but can flourish in deeper muddy parts of the shelf, burrowing through sediment of mudstone to water than can codiaceans. Filamen tous blue-green and dasycladacean wackestone texture. One of the best examples of mollusk zonation in the algae are also present in normal geologic past is that of the rudists in marine environments but are more tolerant of salinity variation and are Cretaceous carbonates throughout the commonly found in water of less than Tethyan Belt . Caprinids and radiolitids lived on the outer to mid 5 m . Ginsburg and others ( 1 972) and dle shelf, monopleurids thrived in Wilson ( 1 975) show the distribution argillaceous sediment of the middle of various algal types across car bonate shelves .
Similar schemes exist for
shelf, and requienids and oysters lived
mainly on the inner shelf.
foraminifera and mollusks.
Figure ?-Nodular bedding in Orbitolina wackestone from Safenia zone of the Cretaceous Glen Rose Formation, Seco Creek, Medina County, Texas. The ir regular beddi ng results from solution· compaction along numerous tiny solution seams concentrating clay layers be· tween purer calcium carbonate lumps.
structures related to surfaces of ex posure or to types of cross-bedding . Bedding Types
Irregular bedding (Fig . 3) i s typical o f shelf sedimentation . Bedding thicknesses vary from thick or
Planktonic foraminifera such as
Sedimentary Structures
Globorotalia and Globigerina occur
in basinal deposits from Cretaceous
A wide variety of sedimentary structures occurs in carbonate shelf
time onward, whereas benthonic
environments. The few that are
foraminifera occur in shelf en
specifically diagnostic of the environ
vironments and are larger, more
ment in which they formed include
massive homogeneous strata to thin ner layers less than 30 em . Shale in terbeds are common. Beds of shelf sediment may be tabular and retain a
constant thickness over large areas, or they may gradually thicken or thin
Middle Shelf
Fig u re 8-Sa m e as Figure 7. C l ose r view, showing abundant Orbitolina.
303
Figure 9-Pianar sedimentary lamination in shallow subtidal to intertidal Upper Cambrian strata in Canadian Rockies above Ice Field Chalet, Banff-Jasper H ig hway.
over distances of several tens of meters. Local variations in rates of sediment productivity and/or transport result in irregular , lens shaped beds. Thin clay partings cause bedding planes and separate units of presumably uninterrupted sedimenta tion. Minor hiatuses then are confin ed to periods of fine-grained ter rigenous sedimentation which seem ingly occur between spurts of more rapid carbonate deposition. Vertical sequences of irregular bed thicknesses then must result from irregular episodic introduction of fine-grained argillaceous material onto the shelf. Contrast this irregularlity of bedding with the well-known rhythmites of certain basinal areas (Fig . 4). Wavy bedding is very common in shelf environments and consists of simple undulose planes at the top and Fig ure 1 0-Sedimenta ry lami nation bottom of beds (Fig. 5). This type of caused by algal stromatolite in shallow bedding grades into nodular bedding subti dal to intertidal strata of uppermost Lower C retaceous Cupido Formation , which displays undulose patterns throughout the bed, giving a n internal Canyo� San L?renzo near Saltillo, CoahUila, Mex1co. nodular or lumpy appearance (Fig. 6) . Nodules and matrix material may be all carbonate or of different composition (for example, limestone nodules in shale) . resist compaction. The resulting tex Flaser bedding, also called sedimen ture is an irregular, closely spaced, almost nodular structure caused by tary boudinage or ball-and-flow structure, is the result of differential the disruption of layers by solution and compaction giving the impression compaction and solution in in homogeneous mixtures of shale and of stretching and flowage. Microstylolites are present at contacts limestone (Figs. 7, 8) . The shales between clay seams and carbonate compact readily, but carbonates masses indicating that the solution of which lithify early in their history
CaC03 may be an attendant process . Inhomogeneity in the original sedi ment may be attributed to burrowing, the presence of shell beds, or local ac cumulations of pebbles or intraclasts on the sea floor. Laminations form in quiet water as sedimentary particles fall from suspension . They develop best in clay aQd silt-size sediment , but are found in sediments of all size ranges . No in ferences can be made as to water depth, for quiet-water deposition can occur in very shallow as well as very deep water . However, in most shelf environments, laminations are destroyed by burrowing. Figures 9 to 1 2 show a variety of laminations associated with shallow subtidal to in tertidal environments. Algal stromatolite refers to a type of plant-induced curvy lamination that occurs mainly in inner shelf to supratidal sediments. Modern species of blue-green filamentous algae (Schizophytes) thrive at the top of the intertidal zone; they also extend downward into near-shore subtidal environments. Actually, no calcified skeletal material exists in stromatolite-forming algae. Studies of Holocene species indicate that these algae form intertidal mats and, at ebb tide, trap fine-grained sediment be tween their soft extended filaments . In order to photosynthesize, every day the filaments grow through the
304
Middle Shelf
Figure 1 1 -Sedimentary lamination in cluding a g raded encri n ite bed, in shallow subtidal st rata, lower part of a n upward shoal ing cycle i n Lodgepole (Mississippian) Formation of Timber Creek Canyon , Big Snowy Mountains, Montana.
Figure 1 2-Aigal stromatolitic lamination overlain by shallow subtidal bu rrowed u nit in El Abra shelf facies of middl e Cretaceous e x posed at Cuesta de El Abra, 20 km east of Valles, San Luis Potosi , Mexico. Shadow at top is hammer.
fine layer of trapped sediment and form new layers . Patch reefs, bioherms and mud mounds affect bedding by building structures that interrupt tabular or wavy bedding. For patch reefs , the massive unbedded reef framework provides the source of sediment that is eroded to form an apron of reef flank material (Fig . 1 3) . These beds are inclined as they onlap the sides of the reefs; dips average about 25 ° but may be as steep as 45 ° . Beds that cap the reef thin over the reef crest and
characteristically consist of a specialized microfacies adapted to conditions of periodic exposure. An example is the common cap of cor nuspirids which forms a dense tangled network of tubular foraminiferal boundstone on top of Late Penn sylvanian and Early Permian bioherms in southern New Mexico and west Texas. Mud mound ac cumulations in Paleozoic and Mesozoic strata interrupting shallow subtidal bedded lime wackestones are illustrated in Figures 14 and 1 5 .
Cross-bedding refers to inclined sets of sedimentary himinations caus ed by wind, wave , and current energies transporting sediment. In carbonate shelf environments, the common cross-bedded rock texture is a grainstone commonly forming in sand bars (Fig. 1 6) . The dominant particle types are ooids, peloids, crinoidal particles or other bioclasts (Fig. 17). Festoon cross-bedding forms as medium-scale cross-beds in sets up to one-half meter thick (Figs. 18, 1 9) .
Middle Shelf
305
Figure 1 3-Patch reef core of caprlnld boundstone overlain by gently dipping flank beds of rudist debris and other bioclasts. Rudist mound along Red Bluff Creek, Bandera County, Texas.
They are caused by the scouring of a channel and its subsequent fill by megaripples in moderately strong cur rents. Festoon cross-bedding is char acteristic of channel deposits and oc curs in outer shelf environments in channels leading down the slope and also in inner shelf environments in tidal channels or in nearshore sand sheets or belts formed by longshore currents (Ball, 1 967). Tabular, foreset , or accretion cross bedding forms by the addition of sediment on the downstream side of megaripples. This type of cross bedding is typical of advancing sand bars (Fig. 20) . Ripple marks are the expression of cross-bedding at the top surface of a bed (Fig. 2 1 ) . They are of two types: (1) Symmetrical ripples have sym metric profiles and parallel, linear crests. The pattern is li ke a sheet of corrugated metal. Oscillation ripples are symmetric and form in shallow water by the in-and-out motion of bidirectional waves on sandy bot-
toms. (2) Asymmetric ripples have asymmetric profiles with short steep slopes on the down-current side and long gentle slopes on the up-current side. Current ripple mark s (Fig. 22) are asymmetric and form by unidirec tional currents in either shallow or deep waters. Exposure Surfaces
Sedimentary structures associated with disconformities yield a variety of features reflecting weathering pro cesses, early diagenesis by fresh water, or the availability of hard substrates for nearshore marine organisms - that is, subtidal hard grounds which are surfaces of non deposition over long periods of time (Figs. 23 to 26). At the time of for mation, these structures occur mainly in inner shelf environments and also on emergent shoals and reef flats across the shelf. However, as regressi ve cycles develop, they are formed on exposed shelf deposits. Collapse breccias form by the
dissolution of more soluble lithified beds and the ensuing collapse of les� soluble strata. Many collapse breccias are formed by evaporite dissolution , but the dissolution of limestone may be equally important . Collapse brec cias are commonly developed on the tops of exposed mud mounds although some brecciation may have formed by subtidal slumping. Figure 27 shows a bedding-plane view of limestone breccia persistently seen at tops of massive open marine middle shelf limestones in Pennsylvanian strata of northern Mexico and southern New Mexico (see also Fig. 2) . Bored surfaces are created mostly by mollusk s and worms that chemically dissolve lithified carbonate rock (Figs. 23 through 25). Usually bored surfaces are best developed along rock shorelines, but they may occur anywhere across the shelf where hard surfaces are available. Similarly, encrusting organisms re quire a hard substrate on which to at-
306
Middle Shelf
Figure 1 4- Lime and mound i n Late J u rassic strata in railway cut. Le Roudel near St. Sulpice in J u ra Mountains of western Switzerland. Transitiona l strata between Effinger marls and Natlca marls.
Figure 1 5-Small lime mud mound about 5 m thick imbedded i n th i n bedded encrinites flanking a much larger mo und. Upper and east branch of Deadman Canyo n . From Lake Val l ey Group (M ississippian) of Sacramento Moun ta ins, near Alamogordo, N ew Mexico.
tach themselves. Certain species of bivalves, corals, and algae behave in this manner, oysters providing the best example (Figs. 25, 26). Clear desiccation features such as mud cracks and curled mud chips, and hailstone and raindrop imprints, are obvious indicators of subaerial ex posure and are rare to absent in open shelf limestone. Planar corrosion zones form by the physical abrasion and truncation of surfaces by marine currents and bioerosion, or by meteoric water dissolution . Such surfaces form in a
variety of ways and may occur in ex posed parts of the inner shelf or as a consequence of submarine erosion in shelf environments (Figs . 23, 24). Burrows and bioturbation are the most common sedimentary structures observed in limestone of ancient shelves (Fig. 28). In fact , the normal shelf wackestone has a thoroughly bioturbated structure. Burrow mot tling of anastomosing trails is marked by color or textural differences , or by dolomitization. Burrow tubes filled with coarser and more permeable sediment are preferentially dolomitiz-
ed (Fig. 29). Isolated burrows and burrows with nested pellets are readi ly recognizable, and the mere presence of discrete burrows probably indicates less burrowing activity than normal. Anastomosing burrow trails are more commonly observed in sub tidal sediment (Fig. 30), and vertical burrows occur more in hardened in tertidal areas. The net effect of bur rowing is to homogenize sediment, destroy primary sedimentary struc tures, oxidize buried layers, and remove organic material from the sediment .
Middle Shelf 307
Fig ure 1 6-Lime sand bars in upper Ain Tobi Formation (Middle Cretaceous) in the section at Tarhuna, Jebel N efusa, near Tripol i , Libya_
Figure 1 7-Small sand bar of fusu l i n id
echi nodermal g ra i n ston e Leavenworth ,
l i mestone, upper Pennsylvanian of Oread megacyc lothem, Osage County, Oklahoma.
Figure 1 8-Festoon cross-bedded ool i l e of Lower Stanton Limestone (upper Pennsylvanian), Tyro quarry, Kansas. Sets are about 8 in thick.
A number of secondary or late diagenetic structures (such as stylolites, fractures, and certain calcite fillings or geopetal cavities) oc cur commonly in shelf limestones. These are not discussed here, because they are not depositional features, and are not specific to normal marine, middle shelf environments .
HOLOCENE EXAMPLES OF SHELF FACIES
Modern carbonate shelves do not offer as much similarity to deposits on ancient shelves as might be liked; basically what is missing in Holocene shelf deposits is the vast expanse of fossiliferous lime mud seen in the geologic record . The sporadic and periodic Late Pleistocene glaciations and non-equilibrium conditions ex-
isting at present (after the drastic and sudden post-Wisconsin sea level rise of more than 1 00 m in 1 8 ,000 years, or about 5 m per 1 ,000 years) indicate that carbonate facies patterns on most modern shelves are atypical . In deed, certain broad shelves are merely veneered with a thin coating of relict calcarenitic sediment . This has been demonstrated for the Campeche Bank, the west Florida Shelf, and the Persian Gulf. The short period of
308
Middle Shell
Figure 1 9-Cross-bedded (medium scale) festoons i n oolite-e ncrin ite strata in sand bar at top of u pward shoaling cy c le (TC 290) in Lodgepole Formation, Timber Creek Canyon, Big Snowy Moun· tains, Montana.
Figu re 20-Fo reset bedding off front of large sand bar, Oolite Blanche Formation in Massangis quarry. Lighte r colored, horizontal beds above foresets comprise Comblanchien Formation, restricted m a r i n e strata, at top of Callovian Middle Ju rassic cycle in Paris Basin.
as the protective Pleistocene Keys, the localization of mud banks, and the special biota of Acropora reefs, may limit analogies with most ancient shelves. Another problem in making com parisons is that most well-known modern carbonate environments (ex ceptions being the Persian Gulf and Shark Bay off Western Australia) are strongly influenced by open ocean conditions with periodic seasonal storms, definite windward/leeward directions, and more open marine cir culation . 5 ,000 to 7,000 years since sea level stabilization has been insufficient for much infilling and covering of the recently transgressed shelves. Yet parts of modern carbonate shelves are very useful in the recognition of an
(Wilson , 1974, 1 975) seen on car bonate shelf margins (a drop-off model), inasmuch as semi-restricted to normal marine muds occur in Florida Bay (inner and middle shelf), normal marine skeletal muds occur
cient sedimentary environments, as
outside the Keys in Hawk Channel
well as in studying process/response
(inner part of the outer shel f) , marginal lime sands occur i n White Bank (outer part of the outer shelf), and coralgal boundstones occur along patches at the shelf edge drop-off in to the basin, the Straits of Florida (Fig. 3 1) . But certain features, such
models. This is true despite the fact
that modern shelves may contain special features caused by the parox ysms of Pleistocene sea level change. South Florida, for example, conforms with the standard facies pattern
The Great Bahama Bank: Drop-Off Model, Offshore Bank
Little carbonate mud is deposited at present on banks open to oceanic influences. This is illustrated by the site of several classic studies of car bonate bank facies, the Great Bahama Bank (Fig. 32) . A fringe of high energy, normal marine bioclastic grainstone (coralgal facies) or oolite grainstone surrounds the bank. Across the broad expanse of middle shelf environments on the bank , the
Middle Shelf
309
Figure 21 -Extensive rippled surface developed on top of a mol lusk fragment grainstone. Highway 36 q ua rry Edwards Limestone (Middle Cretaceous}, Bell County, Texas. ,
Figure 22-Small-scale ripple cross lamination (TC-240} in fine grained peloidal carbonate sand in lower part of upward shoaling cycle, Lodgepole For matio n , Timber Creek Canyon Big Sn owy Mountains, Montana. .
pelletal grainstone/packstone facies occurs (grapestone facies of Purdy, 1 961). This exists even though the water is generally of normal marine ""linity (except where the pelletal packstone/wackestone facies is being deposited in the lee of Andros Island). Circulation is sufficient to transport interstitial carbonate mud to the tidal flats of western Andros or westward in the opposite direction, off the bank. Sediments of the inner and middle shelf environments contain mostly non-skeletal material. The bulk of this facies consists of pellets, peloids, lumps, aggregates , indeterminate grains, and carbonate mud . A very sparse fauna and flora (accounting for no more than 300Jo of the sedi ment) includes mollusks, Halimeda, peneroplid foraminifera, and other foraminifera. In contrast , during the Pliocene, this offshore bank was covered by skeletal packstones and wackestones containing mollusks and foraminifera. Beach and Ginsburg (1980) attribute the Holocene decrease in organic productivity across the
bank to inhibition of growth caused by cooler water and inhibited circula tion due to lower sea level during Pleistocene glaciation . The combined effects of Pleistocene topography and Holocene wind directions are important con trols on modern carbonate facies pat terns. A prime example is the rim of built-up Pleistocene eolianites on the windward side of the bank, as seen in the Exuma Islands, a discontinuous chain of Pleistocene dune ridges formed at the shelf edge of Exuma Sound. With the Holocene rise in sea
level , saddles between the crests of the dune ridges have been flooded, and now tidal currents pass through these breaks. The bottleneck effect here causes swift currents and agita tion sufficient for oolite deposition in the form of tidal spits and bars . The controlling factor of paleotopography on modern sands is clearly demonstrated here. The distribution of potential reservoir facies in the Exumas is parallel to the shelf edge, but in a discontinuous lobe pattern that occurs only in passes between the Pleistocene islands.
310
Middle Shelf
i
Lower unit of couplet
d isconformity
Upper unit of couplet
Figure 24-Po lished slab h a rdground on
Figure 23-Hardground a n d brecci ated s u rface below ba l l -an d -f l ow , nodu lar, or flaser structure in Lower Cret ace ous strata of Thamama Group in a Shell well of l d el Shargi Field, offshore Qat a r .
Belize: Drop-Off Model
The western Caribbean coast from Guatemala to northeastern Yucatan is fringed by a narrow and generally open marine shelf, at most 30 to 40 km wide and as little as 5 km across. Off Belize, the shelf is divisible into several belts (Fig. 33). The outermost is a well developed, thriving barrier reef whose reef flat sediments of coarse sand and rubble are nearly built up to sea level. Shoreward of this, in water depths of a few tens of meters, is a platform up to 35 km wide with numerous patch reefs . The interreef areas are being filled with aragonitic muddy sands. Farther shoreward , in deeper-water lagoonal environments, foram molluscan wackestones and mudstones are being deposited. The finer fraction of this sediment con tains abundant coccoliths washed across the outer shelf from the open Caribbean. Water over the entire shelf is normal marine, except close to shore where effluent from the Belize River moves southward , and in Chetumal Bay behind Ambergris Cay,
of subtidal
c o ra ll i ne l i mestone bed in
Late Triassic Koessen strata at Gaissau se c t i on nea r Adnet, Austria. Notice multiple borings i n l ith i f ie d rock and geop e tal f i l l i n g of cavity below su rface (c ourtesy H. Zankl, Marburg U n iv.). Cor all ites are about 1 em in diameter.
an effective barrier north of Belize City. Here brackish waters with restricted circulation contain very limited biota. The steep drop-off profile along downfaulted blocks (Glovers Bank, Turneffe Keys , and Cinchorra Bank) and the high energy, open marine en vironment at the shelf edge are basically similar to the south Florida model. The present carbonate pattern is superimposed on a Pleistocene karst surface (Purdy, 1 974). The model demonstrates that with open sea circulation and major storm pat terns, most sedimentation remains normal marine and strongly calcarenitic. The only carbonate mud accumulation occurs in the very pro tected, restricted marine lee areas of the shelf. These limited shelves behind coastal reef-sand barriers and high energy carbonate fringes are hardly analogous to widespread sheet deposits of normal marine wackestones found through much of the geologic record. However, they are very similar morphologically to the curvilinear fringe of Smackover grainstone and associated facies around the Gulf of Mexico .
Figure 25-Top view of extensively bored Cretaceous hard g round surface o n oolite with encrusting oysters. Holes are Lithophaga bo rings. Su rface is limonitized. Whitestone Quarry, Cedar Park, Travis County, Texas. Hammer h a n dle is 1 2 em long.
Persian Gulf: Ramp Model
The Persian Gulf is the only ex isting shallow inland sea in low enough latitudes and with clear enough water to permit carbonate deposition (Fig. 32). This area is a large ( 1 ,000 km by 350 km) shallow sea (20 to 80 m deep) almost com pletely land locked in a totally desert climate, largely protected from oceanic influence. A hydrographic barrier caused by upwelling at the Straits of Hormuz largely confines connection with the Indian Ocean to surface water . The Gulf has its own
Middle Shelf
31 1
Fig u re 26-Bored and limonitized �ur· face at hardground on Ju rassic Callovian strata. southeast Paris Basin, France. Note blac kened nodules on surface.
F[gure 27-Top view of i rreg u lar blackened clasts (?) bel ieved to result from exposure and partial desiccation. Such surfaces cap grainstone-packstone at top of major upward shoaling cycles in middle Pennsylvanian shelf strata of southern New Mexico and northern Chihuahua. Staff is about 30 e m long.
moderately strong tides with a range of several meters . Minimal ingress of fresh water occurs at the northern end. The Tigris, Euphrates, and Karun Rivers flow only part of the year and do not bring much fine grained terrigenous material into the basin. The sea bottom is markedly asymmetric with a deep Oess than 100 m) axis close to and paralleling the Iranian coast . The vast expanse of the
southeastern Gulf (about 200 km
across) is the shallow Great Pearl Bank which geographically ap-
proaches very closely some ancient in land epeiric seas. Its depth ranges from 10 to 50 m . Surface salinity here is about 39 to 400Jo . Salinity increases slightly with depth across this shelf. Tectonically, the Persian Gulf is a foredeep area fronting the Zagros mobile belt; its carbonate shelf is built out from the Arabian Shield. This shallow shelf was apparently laid bare by Late Pleistocene sea level lowering. In the southeast portion of the Gulf, the bottom topography is controlled by westward-facing cuestas
of Eocene and Miocene strata and by Pleistocene karst drainage and ter races . However, regional dip is less than 0.5 m per kilometer, so the sur face is essentially flat. The embayment southeast of the Qatar Peninsula (offshore Abu Dhabi-Dubai-Sharja) has a veneer of various carbonate sediments. The slight swales contain carbonate mud accumulations; the higher areas are shoals of bioclastic grainstones (main ly bivalve fragments), muddy packstones, and coralgal grainstones.
312
Middle Shell
Figure 28-Burrows in arg i llaceous limestone of Jurassic shelf strata (lias 5 beds) in Valley of Siz River, H igh Atlas of south-central Morocco.
Fig u re 29-Burrow-controlled dolomite and ferruginous mottling on top of Khumi (Lower Cretaceous) l imestone in Tang+ Gurguda, Zagros Mountains, I ran.
The grainstones are presently being Depth of water on the open shelf, in cemented on the sea floor. Crudely which wave action can winnow mud defined facies belts extend out from and can round sand-size bioclasts, depends on the degree of protection the Qatar Peninsula eastward across by the Qatar Peninsula, which shields the northern part of the Great Pearl Bank toward the axis of the Gulf the area from the northwest shamal some 200 km offshore (Fig. 32) . wind and inhibits wave action in its Water depths increase from a few lee. This depth generally varies from meters near shore to about 50 m out 10 to 20 m. Holocene sediments are into the Gulf. generally thin across this area, In sequence from shallow to deep, averaging about 2 m thick, with 10 to these facies are: rounded mollusk 15 m thickness present in a few low fragment and gastropod grainstones, areas as determined from sparker profiles . The fauna is essentially nor angular molluscan grainstones, molluscan wackestones and mal marine along this traverse packstones, and dark argillaceous car (Houbolt, 1957), but lacks bonate mudstones and wackestones . globigerinid foraminifera and codia-
cean algae. In a general way, the facies pattern of the southeast Persian Gulf is spread out along a very gently sloping ramp-one that is almost flat but with local irregularities. Some finer grained sediments are trapped in coastal lagoons, but for the most part, coarse-grained sands are deposited close to shore, and bioclastic wackestones occur farther offshore. Fringing reefs are beginning to be established in some places close to shore and along windward edges of some shoals, but time has been too short and subsidence too slight since the end of the last transgression
Middle Shelf
313
Figure 30-Cemented Calianassa-type burrows in the Globigerina Limestone (Miocene) at Fomm-ir-Rih Bay, Malta.
Figure 3 1 -Landsat image of southern Florida, Florida Bay, and the F l o r ida Keys. The main landmass shown is the southern tip of Florida, with the Everglades veg et a t io n appearing red. Field checks indicate that l ight b l u e in image corresponds to carbonate sediments in very shal low nearshore (less than 1 0 ft deep) to supratidal en vironments; medium blue corresponds to subtidal carbonate environments (less than 20 ft deep); very dark blue to black corresponds to deep ocean waters in the Straits of Florida which occupy about 2/3 of the area show n . Clouds a re white and thei r sh adows black in the im age. T h e d i stance represented across the photo Is approximately 1 1 5 m i .
(5 ,000 to 7,000 years ago) for a car bonate platform margin (drop-off model) to be established at the gentle break in slope of the bank about 200 km offshore. SHELF FACIES THROUGH GEOLOGIC TIME
The control of biological evolution on carbonate shelf facies through geologic time is indicated by the fact that certain shelf facies are very char acteristic and occur in strata of the same age around the world. For ex ample, there are the famous Jurassic oolite grainstones from the U.S. Gulf coast, northwestern Europe, and the Middle East ; Cretaceous rudist
boundstones from the Tethyan seaway and the Gulf of Mexico; Mississippian crinoidal grainstones from the midcontinental U.S.A., Alaska, and western Europe; and glauconitic sandstones and trilobite hash beds from the Cambrian. Early Paleozoic During this time, carbonate shelf facies consisted mainly of bioclastic grainstones, -
packstones, and wackestones (Figs . 34, 35); oolites were common, especially in the Cambrian of North America. Normal marine conditions were widespread in the Early Paleozoic, and shelf bioclasts were dominated by brachiopods, bryo zoans, pelmatozoans, and trilobites, the latter being particularly abundant in the Cambrian and Lower Ordovi-
314
Middle Shelf
� �
D Tide flat
M uddy bioclast
Peloids sand
Oolites
Figure 32-COmparison of modern ca rbonate facies of Persian Gulf G reat Pearl Bank (ramp model} and Great Bahama Banks (drop-off model) on opposite page.
cian. Patch reefs and micritic mounds before Middle Ordovician time con tained an assemblage of sponges, calcareous algae, stromatolites, and gastropods. Middle Paleozoic - Shelf facies at this time were similar to those of the Early Paleozoic, except that in Later Ordovician, Silurian, and Devonian times, rugose corals were added to the brachiopod-bryozoan-pelma tozoan-trilobite faunas (Figs . 36, 37). All of these indicate normal marine
salinities across the shelf. It is in teresting that the original mineralogy of these dominantly Paleozoic faunas was all low-magnesian calcite, except for echinoderms which are high magnesian calcite. Aragonite-secreting organisms are hardly represented as fossils except for nautiloids, gastropods, and quite possibly stromatoporoids and receptaculitids. In the Middle Paleozoic, these presumed aragonitic forms were most common in and around carbonate
build-ups such as mud mounds and patch reefs . Across the North American craton, the textures of these shelf deposits are mostly micritic, occurring commonly as skeletal wackestones or even as calcareous marls. Both generally con tain whole fossils. The patch reef facies consist of boundstones with bryozoans, tabulate corals, rugose corals, receptaculitids, and stromatoporoids (Fig. 38) . The diverse biota in these reefs, their im-
Middle Shelf
31 5
1 00 Kilometers Depth contours in fathoms
Reefy
Bioclast sand
pressive dimensions, and growth potential contrast strikingly with those of the earlier Paleozoic. Late Paleozoic In Mississippian and Early Pennsylvanian time, outer shelf facies were dominated by stalked pelmatozoans (crinoids and blastoids) and bryozoans to the exclu sion of most other invertebrates (Figs. 39, 40). Platy green algae, fusulinids, and other diverse and abundant foraminifera were the main additions to Early and Middle Paleozoic shelf -
Lime mud
facies from Pennsylvanian to Permian time; they are common faunal elements in micritic carbonates and marls spread across the shelf (Fig . 42 a through f). Patch reef faunas con tain numerous encrusting organisms such as certain tubular foraminifera, algae, hydrozoans, and several promi nent forms of uncertain biological af finity (Tubiphytes, Fig. 4 1 ; Colinel/a,· Archeolithoporella). Corals and stromatoporoids declined noticeably in the Late Paleozoic. Mississippian
La nd
and Early Pennsylvanian patch reefs contained bryozoans and the corals Chaetetes, Lithostrotionella, and Sy ringopora, but these reefs were generally small . All of these corals apparently thrived in muddy en vironments Pennsylvanian and Early Permian build-ups were formed by phylloid (platy) algae in wackestone textures (Figs. 42f, 43, 44) ; beds flanking the mounds were typically crinoidal or consisted of other bioclastic packstone or grainstone .
316
Middle Shelf
() Figu re 33-Belize shelf margin drop.off model showing facies belts .
debris derived from communities on top of the mounds. Later Permian reef biota are very distinctive, con taining many encrusting algal and algal-like forms, hydrozoans, and a large array of sponges. These faunas continued well into the Triassic. Mesozoic Characteristic Mesozoic shelf facies are molluscan wackestones with abundant echinoids and codiacean and/or blue-green algae ( Cayeuxia, Lithocodium, or Bacinella) . In the Early Mesozoic,
-
crinoids and brachiopods were also still abundant . Large shelf foramin ifera became prominent , with large valvulinids and pseudocyclaminids in the Jurassic and older Cretaceous, and Orbitolina and alveolinids (Figs. 45 , 46, 47) later in the Mesozoic. Molluscan and coral biostromes are common in Mesozoic strata and are illustrated in Figures 48 to 5 1 . The Cretaceous also has patch reefs which, early in the period, consist of corals and stromatoporoids and later
of various rudist boundstones with flanking strata of reef-derived debris. From Cenomanian to Danian time, another characteristic outer shelf facies is pelagic coccolith chalk. The same facies with much more diagenetic alteration is known from the Tethyan seaway at the Jurassic Cretaceous boundary; the Solnhofen, Oberalm, Biancone, and Maiolica limestones . Tertiary Skeletal wackestones dominate the shelf facies of the Ter-
-
Middle Shelf
31 7
Figure 34- Lower Ordovician microfacies con sist i ng of cri noid-trilobite intraclastic grainstone, occurring in small channels between algal bi oherms Sample AGG, El Paso Limestone, Franklin Mountains, El Paso, Texas; plane polarized light .
Figure 35-Middle Ordovician microfacies. Brach iopod packston e, bioclastic hash partly infilled with lime mudstone. Salona Limestone i n central Appalachians, Pennsylvania, 6X. (bar scale 2 mr:n). =
tiary with abundant mollusks, echinoids, and large foraminifera such as Nummulites and lepidocyclinids (Figs. 52 to 55). These large forms constitute a special outer shelf shoal facies of grainstones and packstones. Patch reefs and biogenic banks of corals, red algae, and bryo zoans represent a cooler, more temperate climate in parts of the Mediterranean , more during the Ter-
tiary than in the Cretaceous. The universality of Eocene and Oligocene coral faunas change in the Miocene to more provinciality. Pleistocene Following the Holocene rise in sea level, Pleistocene shelves were generally exposed only at outer shelf edges where heaped up eolianites or rapidly growing marginal reefs formed. This is the case in Florida, the Yucatan, Bermuda, and -
the Bahamas. However, recent dril ling across the Great Bahama Bank (Beach and Ginsburg, 1980) indicates that Pleistocene facies are similar to overlying Holocene pelletal grainstones . Pre-Pleistocene facies, on the other hand, consist of more typical shelf facies of skeletal packstones and wackestones.
318
Middle Shelf
Figure 36-Strata of Late Devonian (F ras n i a n) age between patch reefs near Erfoud, South Morocco, with ammonites and orthocerid nautiloids. Vi ew is about 1 m across.
Figure 37-Middle Pa leozoic shelf shallow basin microfacies. Skeletal wackestone with crinoids, brachiopods, Umbel/ina fragments from the Tenneco, Tange 1 in northeastern Montana is the open ma rine phase of typical Duperow (Frasnian) cycles.
ANCIENT EXAMPLES OF SHELF FACIES
A few regional facies studies of shelf sedimentation with sufficient detail for satisfactory interpretation are available. Several of these are discussed below. Leaven worth Limestone - In Late Pennsylvanian (Virgilian) time, widespread shelf carbonates were deposited as thin sheet limestones across the mid-continent of the United States (Fig. 56) . These limestones are constituents of typical Carboniferous cyclothems, represent ing chiefly the inundative phases
when maximum spread of the seas oc curred. The complex meandering shoreline trends roughly north-to south, and gradual facies changes in the sheets of limestone are east-to west (Wilson , 1 97 5) . Because the strike of the outcrops is roughly NNW-to-SSE and only slightly obli que to the depositional strike, very uniform facies sequences may be traced in Virgilian strata from Iowa and southern Nebraska to northern Oklahoma. Detailed studies by Toomey ( 1969a, b, 1 972) on the 1 m-thick Leaven worth Limestone of the Oread megacyclothem describe three shelf
facies and the distribution of algae and foraminifera. Sample control is from 29 stations along the outcrop belt and all locations are in middle shelf facies. The eastern shore strata are eroded, and the offshore subsur face extent has not been well studied lithologically. Skeletal wackestones (Toomey's skeletal mud facies) com prise the dominant shelf sediments and occur throughout the outcrop belt. Two additional facies were en countered but only at the extreme northern and southern outcrops: ( 1 ) skeletal wackestones associated with mudstone and (2) onkoidal packstone to wackestone and fusulinid bioclastic packstone - termed the aggregate grain facies (Fig. 1 7). From the nar row strip of outcrop, one assumes the broad irregular facies pattern typical of wide shelves . Toomey's distribu tion charts demonstrate that most algal and foraminiferal species are uniformly spread across the shelf and show irregular patterns of distribu tion . The presence of Epimastopora (a dasycladacean alga) indicates
Middle Shelf
319
Figure 38-Middle Paleozoic stromatoporoid boundstone from a patch reef in the Duperow Formation (Fras· nian) of northeastern Montana.
Figure 39-Crinoid·bryozoan grainstone from a bioclastic bank or shoal in the Lodgepole Formation (Mississippian} shows micrite rims around crinoid and blastoid particles and shows rounded, abraided bryozoan fragments. Timber Creek section, Jenks 1 · 1 , Big Snowy Mountains. central Montana.
shallow water depths of hardly more than 5 to lO m along the whole 500-km width of the shelf. In summary, the Leavenworth Limestone is a uniform sheet, 500 km long and at least many tens of kilometers broad, trending into the subsurface of central and western Kansas . It shows second and third order variations that occur in middle shelf environments, but such facies patterns are irregular with a non-
systematic trend. Associated limestones in the Oread megacyclothem, like the Toronto and Plattsmouth, show similarly uniform patterns . Hueco Limestone The Hueco Limestone of Lower Permian (Wolfcampian) age and its equivalents throughout southern New Mexico, west Texas, and northern Mexico il lustrate a gently inclined ramp profile across the Orogrande Basin (Jordan -
and Wilson, 197 1 ; Jordan, 1 975). Figure 57 shows the paleoen vironmental setting during the Upper, Middle, and Lower Wolfcampian. In contrast , carbonates deposited on the shelves surrounding the Pedregosa Basin change facies rapidly, forming a steeply inclined shelf margin (drop off model) in southwestern New Mex ico and in northern Chihuahua. Normal marine skeletal wacke stones and packstones form the bulk
320
Middle Shelf
Figure 4 0a-Encrioite-pockstone ot crinoi&.J fro.grnents and tritl.irated
brsohlopod·bryo�oan debris. 6X. from b�ds flanking lime mud mounds of Lak� Val ley Gr·:xJP (Mississippian) of Sacramt:onto MO<..ln!:ains, r-l�w M 9.¥ 1CO. (�c::�l� bar = � mm).
Figure 40b-Cri n o id·bryozoan � ral n stone slrata of southeEI.Stern M issouri.
from Mississippian
Middle Shelf
321
Figure 4 1 - Tubiphytes encrusting organism. 8X. from Townsend reef trend, Shell Lusk No. 1 , 1 0,626 ft. Lower Per mian (scale bar = 1 mm}.
of middle shelf sediment (Fig. 42 a to f). Fossil material includes whole and fragmented crinoids, brachiopods , algal plates, large foraminifera (especially fusulinids and endothyrids), bryozoa, pelecypods, gastropods, ostracods, and trilobites. Peloids are also common consti tuents; they were most likely soft pellets formed in place by sediment injesting burrowers and detrital feeders. Typically, these middle shelf facies exhibit some degree of burrow ing. Shoal environments are best developed in Upper Wolfcampian time in west Texas (Hueco Mountains and Sierra Diablo) . Bioclastic · grainstones and packstones, especially rich in large benthonic foraminifera, were deposited on a broad, submer ged southwestward extension of the Pedernal Landmass. Obvious round ing of grains indicates high energy, shallow water conditions; very shallow waters Oess than 20 ft) are further substantiated by the occur rence of dasycladacean algae fragments. A second type of shoal deposit is the cross-bedded fusulinid grainstone or packstone facies. The elongate, football-shape of the fusulinid tests, along with their low bulk density, made them ideal current indicators.
With the proper conditions of current velocity and grain mobility, the fusulinid could pivot and be oriented by currents with its long diameter parallel to the direction of current flow. Patch reef or biohermal facies in the Hueco consist of brecciated algal plate and Tubiphytes wackestones . Algal plate mud mounds formed best in the Middle and Upper Wolfcamp ian in the central part of the Oro grande Basin . Influx of clastics from the north (Abo) and from the east (Powwow Conglomerate) off the Pedernal Landmass probably mud died inner shelf waters, pushing algal mound development out into the shal low water Orogrande Basin . Algal plates acted as baffles for suspended carbonate mud and aided in the trap ping and accumulation of fines. Tubi phytes, an encrusting foraminifer(?)1 (Figs. 4 1 , 42f) attach themselves to algal plates and can form a con siderable part of the sediment. Brec ciation occurs early in the rock's history as a subaerial breccia, formed during exposure and desiccation. As a whole, Hueco shelf facies are best described as a facies mosaic in volving various mixtures and com binations of skeletal and pelletal par ticle types in muddy-matrix car bonates. Included here are crinoidal
packstones, brachiopod-crinoid wackestones , and pelletal-skeletal packstones, to name only a few. Edwards and Glen Rose Limestones Middle Cretaceous (Al -
bian) rocks in central Texas were deposited on the wide, extensive Comanche Shelf (Fig . 58) which covered most of the state. During Ap tian through Cenomanian time, the seaward margin of the Comanche Shelf developed as a shelf edge rudist complex extending in a remarkably even curvilinear pattern along the northern boundary of the Gulf of Mexico from Mexico to Louisiana. This belt is referred to as the Stuart City Reef and consists largely of rudist, coral, and algal debris. The presence of a long, linear thin area in the overlying carbonates and shales, a "thin" which tracks the trend of the reef belt, indicates that the Stuart City Reef formed a ridge with some relief. Immediately seaward of the shelf edge position of the reef trend, the sea floor dropped off abruptly and deeper water carbonate mudstones and wackestones were deposited on the adjacent forereef slope. This well-developed reef and 1 Its origins are undetermined; it has been
classed by various workers as a hydrozoan, foraminifer, and a type of algae.
322
Middle Shelf
b-
d-
tFigure 42-Bioclast!c rnicrofacies of t� Wolicampian Hueco Limestooe. Permian ot southern New Me:
(tubulatJ forarns), staffell ids, and ottner genera. (C} Tub•Jiar foram grainstone, San Andres Mountains {S.� 1 3), N ew Mexico, commonly forms a capping bed on b ioherms. (d) Fusul l n ld
beflthonlc
pa·::: kstO(le, Jarilla M OU(Itai ns. New M6.X· ico; noH:J compactioo effects - deform ed and orushed fusu lini ds. (e) Algal pla.te-ro ra rni n ifera I oackstonefg rainston(;). Hueco Mountains (HM 73), Texa.s. Algae I n c lude Eplmastopora, a dasycladacsa�
indica! ive of very shallow wa ler . (f) Algal �ate·Tub�ohytes wackeotone. Frankl i n MoiJTltains !FM 89). Texas. TutJioflytes (dark br<Jwn) are shown in the 4-l pper left encrustin g a recry.:,."talli.::ed plate of g reen algae; g43ne rally, Tvt>iphyres appear as a detrital grair1 {lower left near the seal!'! bar). separated trom a tom1er1y
crusted S'Jrface.
Middle Shelf
323
Figure 43-Leached alga l plate grai nstone showing codiacean{?) algae in . M iddle Pennsylvanian (Desmoinesian) reefs along the San Juan River. Note porosity development between as well as wit h i n algal plates. Sa mp le SJ-1 28-8.
Figure 45-Lower Cretaceous
��..�'""'�"t''n"' microfacies of miliolid
ifera, echinoderm, and molluscan 25X. M iddle Cupido Fo rmation, Potrero de G arci a north of Monterrey, Mexico, PG-IV-32 (s c al e bar == 1 mm}.
,
Figure 44-:S i l i c ified c odiacea n (?) platy a lgae o n outcrop of Late Pen nsylvanian strata, Horqu illa Fo rma t io n of Big Hat
chet Mountains showing abundance of these accumulations.
the broad shelf to the northwest behind it represent a typical drop-off model of shelf-to-basin transitions. This shelf edge pattern commonly is regressive and, with several variations (Wilson, 1974; Hine and Neumann, 1 977) , occurs throughout geologic time in many stratigraphic sequences. In contrast , the Comanche Shelf represents a ramp model of sedimen tation. This is indicated by a steadily deepening profile of water depths and carbonate facies on the shelf. In the middle shelf environments, broad depressions and swells exerted some influence on thickness and type of carbonate facies deposited . The major
depressions were the south Texas-Maverick Basin and north Texas-Tyler Basin, both of which were filled with evaporites at times in the Albian . Separating these two depressions was a broad elongate swell , the central Texas-San Marcos Platform, bearing southeasterly and seaward from the vicinity of San Angelo. The San Marcos Platform was drowned by the Albian transgres sion. The Albian strata across this shelf contain many thin, discrete, widely correlatable units, part of them exten sively dolomitized ; patch reefs occur both toward the north and southwest perimeter of the dome-shaped Llano Uplift which lies across the axis of the platform (Rose, 1 972; Fisher and Rodda, 1969). The lower half of the Glen Rose, as exposed on the San Marcos Platform (Figs . 59, 60) provides a good exam ple of shelf margin facies. Its inner shelf facies fringing the Llano Uplift is as little as 10 m thick although it thickens to the south; it contains argillaceous beds, algal stromatolites, and ripple-marked pelletal-molluscan grainstones . Its middle shelf facies is as thick as 100 m and is dominated by skeletal wackestones, marly mudstones, calcareous shales, and localized reefs and biostromes; there occurs one very fossiliferous unit
324
Middle Shelf
Figure 46-Lower Cretaceous ac�(estone microfacies with peloids and id, 32X. Upper Cupido Forma· Potrero de Garcia, north of Monter· rey, Mexico. PG·V·6 (scale bar 1 mm). =
Fig u r e 48-Biostrome of megalodont bivalves uppermost Triassic strata (Rhaetian at Brauneck, Bavaria, North· ern Limestone Alps).
Figure 47-Lowe r Cretaceous wackestone mic rofacies, oyster biostrome, 9X, from uppermost Cupido Formation, Potrero de Garcia, north of Monterrey, Mexico. PG-VI-5 (scale bar = 1 mm).
·
toward the top, the Salenia Zone. Glen Rose patch reefs and biostromes are zoned across the inner and middle shelf according to reef framework with oyster, monopleurid, and caprinid reef facies occurring in northeast-to-southwest trending bands paralleling the early Glen Rose shoreline (Figs. 61 , 62). Figure 63 is a cross section somewhat higher in the Albian (Fredericksburg Group) which trends
obliquely across the shelf of the San Marcos Platform into the north
Texas-Tyler Basin . Over the central Texas High (Llano Uplift), the Ed wards facies contains tidal flat mudstones, dolomites, miliolid grainstones, and requenid biostromes.
The inner shelf facies prograde over the middle shelf, where a series of high energy grainstone bars developed not far offshore. Figure 64 shows the distribution of carbonate facies and reconstructed environments of deposi tion at Moffat Mound near Belton, Texas (Kerr , 1 977) . The bank measures 40 m thick, several km wide, and about 70 km long. It is basically an oolitic grainstone with skeletal wackestones to the north in front of the bank . Farther offshore, circular caprinid rudist patch reefs oc cur, surrounded by chalky wackestone. Behind the bank, lenses of scattered patch reefs, reef-derived sands, and skeletal packstones occur. These facies show regressive develop ment of the mound from normal marine shelf through grainstone shoals and rudist patches to exposed beach calcarenites and restricted marine lagoonal sediments, to inter tidal mudflats and supratidal dolomites (Fig. 65). These same strata are developed southward across the San Marcos Platform to the Stuart City Reef Trend which is another ex ample of the drop-off model presented originally by Griffith and others ( 1 969) (Fig. 66) .
Figure 49-Biostrome of lithiotid bivalves i n Jurassic shelf limestone (Lias 3) of High Atlas, Siz River Canyon , Morocco.
ECONOMIC CONSIDERATIONS
Original shelf carbonate sediments consist dominantly of the minerals aragonite and Mg-rich calcite as car bonate mud with a certain percentage of grains . Generally, initial porosity is high (40 to 70%), and permeability is very low. The mud matrix alters to micrite, which consists of a mosaic of tiny (4 to 10 micrometers) calcite rhombs with much reduced porosity (less than 5 to IODTo) and low permeability (less than 10 md). Many ancient limestones of the shelf facies are completely dense, owing t o a
welding process involving grain solu tion and reprecipitation of calcite on the grains of the originally porous sediment . Only certain depositional environments and special diagenetic
Middle Shelf
325
Figure 50-Biostrome of dendroid finge r corals, base of Middle Ju rassic below pinnacle reefs near Rich, above Slz River Valley, High Atlas, Morocco.
Figure 51 -Biostrome of rudist bivalves
(lchthyosarcolites bed) in middle of Ain Tobi Formation (M iddle Cretaceous),
Tarhu n a section of Jebel Nefusa, near Tripoli, L i bya .
conditions result in preserved porosity and permeability sufficient to form hydroc arbon reservoir rock in middle shelf environments. In view of this, it is interesting that the world's largest single reservoir (the Jurassic Arab D Zone of Saudi Arabia) is in coarse carbonate grainstones of middle shelf facies (Fig. 67) . In the progression of facies across carbonate shelves, potential middle shelf reservoir facies for oil, gas, water, and ore-forming fluids occur in shoal sands, patch reefs, dolomites , and leached horiz;ons beneath unconformities. Widespread pelagic chalks form on deeper shelves and are also middle shelf reservoir targets. They are very porous (up to 450Jo porosity), but due to their very fine grain size, have low permeabilities (less than 2 md). Many shelf carbonates have inef fective porosity attributed to either
modest increase in porosity. Even ex tensively fractured reservoirs show only about a 60Jo maximum increase in porosity due to the storage capability of fractures. Log response - Traditionally in t he petroleum industry, the use of
electrophysical logs has been more ex
bonates - for example, limestone-to dolomite gradations and a wide range of rock types that includes interbed ded or interspersed evaporites, shales, and siltstones; (2) Even pure limestone sequences contain basically inhomogeneous textures - for exam pl e , wackestones grading into
finely disseminated void space in the
tensive and more quantitative in pro
matrix or to isolated moldic porosity from dissolved bioclasts or oolites. In such tight facies, well developed frac tures function as primary conduits for fluid flow with the main enhancement
duction/exploitation situations than
carbonate sections are usually more
in exploration . Generally, interpreta
severe or dramatic than in clastics,
tions derived from log analysis in
since metastable carbonate minerals
of reservoir quality being improved
permeability, accompanied by a
packstones; (3) Diagenetic effects in
clastic sequences are more straightfor
are sus ceptible to alteration by sur
ward than those in carbonates .
face or subsurface fluids. Thus, the
Several factors account for this: ( 1 ) Complex mineralogies occur i n car-
distribution of diagenetically induced porosity can be very complex ; and (4)
326
Middle Shelf
Figure 52a-Typical Tertiary bioclastic wackes1one with large benthonic foramlniiera, Conoco No. 1 Rubah, 4 1 35 ft, I ndonesia; nalural size.
With the recognized variability in car bonate systems, the effects of averag ing the response of thin beds with mechanical logs can be problematical. With these limitations in mind, it is evident that running a suite of logs is superior to depending on any single
log response (Figs. 68, 69) . Having a suite of logs permits the use of Schlumberger cross-plots, as describ ed by Asquith ( 1 979), to determine lithologies . The most cost-effective suite is the ISF Sonic. This suite pro duces a combination of the following
traces: ( 1 ) Sonic log - Used for porosity evaluation (slow zones correspond to porosity development). Sonic log is used to make synthetic seismic traces at the well which are then used to tie in with seismic lines near the well site.
Middle Shelf
(2) Gamma ray log - Useful in in dicating shale beds by their naturally high radioactivity.
(3) Induction log - Gathers
the density log is also run . In car bonate sequences , the main density differences are between limestone
(2.7 1 gm/cu em) and dolomite (2.87
resistivity data for evaluating fluid
gm/cu em). Density logs measure
content and saturations.
bulk porosities with a precision of 1
(4) Spherically focused resistivity
log - Very shallow penetration is
to 2% .
(5) Neutron log - Neutron logs
useful for formation evaluation. Aids
are also commonly used. Combined
in the interpretation of resistivity logs
with sonic and density logs,
with deeper penetration. More quan titative interpretations can be made if
lithologies and porosities can be determined by cross-plot techniques .
327
Figure 52b-Foraminifera! packstone of middle shelf facies of the MokaHam For· mation (Middle Eocene); sampled in Cairo from quarries that once supplied building stone for the pyramids. The larger foraminifera are microspheric forms {calle<:J Nummulites gizehensis), and the smaller ones are megalospheric f"rm<>
/l'<>ll<>ri Mummll /itQ(' f'l l rlli'O'nir.�\
328
Middle Shelf
Figure 53-Red algal colonies 4 to 6 em across with bryozoans in shallow sub· tidal-shelf sediments of Oligocene. Lower Coral l ine Limestone, Blue Grotto, Malta. Fig u re 54-Red algal and large fora m i n iferal fragments i n grainstone sho a ls of shelf facies, 6X, Lower Cor alline Lim estone of Malta (sca le bar = 2 mm}.
Figure 55-Large foramin iferal, bryo zoa ns, miliolid packstone of shelf facies, 6X, Lower Coralline Limestone of Malta (scale bar = 2 mm).
Log responses across carbonate shelves tend to be most uniform in middle shelf environments where broad sheet deposits predominate. Reservoir targets on the shelf include patch reefs, grainstone shoals, and tabular dolomites . Mechanical logs indicate porosity and basic lithologies ; however, detailed facies information is available only through sample examination. For example, recognizing Dunham textures or rna-
jor carbonate particle types cannot be accomplished with log analysis; the same holds true for distinguishing porous boundstone textures of patch reefs from porous oolite grainstones. Obviously, it is necessary to combine both types of information to identify and map carbonate facies with reser voir potential . Reservoir Facies
Linear shoals o f porous grainstone
commonly form in outer shelf en vironments, at or behind the shelf margin and in trends parallel to the margin (Ball, 1 967). In middle shelf environments, shoals are less common and form either as irregular, elongate bars or as broad subcircular or ring like shoals deposited over or around positive areas on the shelf. The Great Pearl Bank of the Persian Gulf con tains such bodies of sediment encir cling weak positive elements across
Middle Shelf
329
'\ �c:,
..
W { b i o clostic wackestone ) A L G A L - B A L L P/W o n d FU S U L I N I D
�, W / M
G
the bank, some of which are growing salt domes. Shoal facies in the inner shelf environment include tidal deltas , nearshore bars, beaches, and dunes . Porosities in Holocene shoal facies range from 40 to 500Jo with an estimated 5 to 1 0% of the total due to intragranular void space. Grainstone bodies of the inner and outer shelf are deposited in trends parallel to the shoreline and shelf margin respectively, and such orienta tion may be of great use in predicting trends of reservoir rock. Shoals of the middle shelf, however, are less abun dant and more complexly distributed . An understanding of former circula tion patterns in a basin may be necessary to predict the distribution of shoal facies in middle shelf en vironments. Such an understanding may be possible only in densely dril led areas . Prolific carbonates of the Arab Zones in Saudi Arabia produce
���' ()\) E N C R US T I N G FO R A M B I OFA C I E S MIXTUR E OF M I X E D FO R A M and F U S U LI N I D 8 I O FA C I E S
from primary porosity in shoal sands of the middle shelf environment, but wide spacing of drill holes and dril ling exclusively along structure in these highly permeable sands has precluded any delineation of trends of component sand bodies . Patch reefs form as steep-sided boundstone build-ups that rise abruptly from a lagoon floor. They may be common in middle shelf areas . Erosional debris from the reef framework forms sands which are trapped as internal sediment pockets and channels within the reef itself. An apron of reef-derived sediment, formed as flanking beds, grades down-dip into bioclastic wackestones of the lagoon. Several types of porosity (see Choquette and Pray, 1 970) combine in these facies to make almost any part of the patch reef system a good reservoir target : cavities constructed and sheltered by
Figure 56-Reg io nal interp retation of fa ci e s of the Upper Pennsylvanian Leavenworth Limestone of Kansas out c rops after D. F. Toomey (1 969).
boundstone growth (SH), intra particle void space in internally porous organic framework (WP), in terparticle voids between grains (BP), and true cavities formed by pillared reef growth (GF) . Exploration in patch reef facies en counters several problems, the first of which is that the origin of patch reef development (what makes a patch reef grow where it does?) is simply not well understood. Purdy ( 1 974) proposed that with a rising sea level irregular paleotopography is com monly encrusted with corals and red algae to form patch reefs . Water con ditions (circulation patterns and salinities), as well as substrate type, affect patch reef development . In some areas, the vagaries of distribu tion of coral spat and larvae may control the ultimate shape of reefs , their spacing, and the general reef trend - making prediction of reef
330
Middle Shelf
Upper Wolfcampian
Middle WoHcampian
Figure 57-Facies of Wol fca mpi a n Hueco Limestone of southern N e w Mex ico and West Texas . Line A-A ' Indi cates a "d rop-off " profile from the shelf to the Pedregosa basi n , whe reas line 8-B ' in· dicates a ramp profi l e i nto the in tracraton i c Orogrande Basin.
location essentially a paleoecological problem . A second problem in patch reef facies exploration is that the term reef is confused in the literature. Follow· ing Dunham (1970), ecologic reefs (having true boundstone textures and a typical reef biota) may be distinguished from stratigraphic reefs whose textures and fossils are largely unknown . Ginsburg (in Wilson, 1 975) has used the general term carbonate build-up to include both types of reef bodies . The Miocene of Salawati, Irian Jaya is a case in point. Several "reefs" recognized on seismic lines as build-ups were also indicated in bore holes by SP deflection and other log responses. Detailed sample examina tion reveals that few of these are true coralgal reefs and that many are muddy biogenic bank deposits, main ly packstones with abundant fragments of coral, red algae, and foraminifera. The texture is definitely detrital rather than the growth framework texture of true bound-
D til •
Clastics Shoal FaciM
Reef Facies
Lower Wolfcampian
stone . The red algae here is the thin branching type, not the typical massive encrusting Lithothamnion of the Pacific, and the foraminifera are large benthonic forms such as Lepidocyclina and Gypsina. A third problem in patch reef ex ploration is that a good paleon tological understanding of the par ticular reef fauna and flora is essen tial to the recognition of reef prox imity indicators. Halos of dispersed reef-derived fragments occur around patch reefs. For example, around Holocene patch reefs on the Bermuda Platform, the concentration of coral and Homotrema (a large encrusting foraminifera) in the sediment in creases near the reef and decreases lagoonward (Jordan, 1 973). But the volumetrically dominant reef flanking material is the marine alga Halimeda. Dolomites form predominantly in inner shelf environments, in trends parallel to the general shoreline. Such dolomites are typical of tidal flat en vironments , with complex facies
variations involving deposits formed in tidal channels , levees, channel bars, beach ridges, ponds, and inter tidal flats. Across some shelves, the inner shelf processes of dolomitiza tion apparently acted at a much larger scale and continued to the ex tent of dolomitizing the entire shelf out to the shelf margin (the Upper Cambrian-Lower Ordovician Knox Dolomite of Tennessee, described by Harris , 1 973). Generally, when fossiliferous sub tidal normal marine facies are dolomitized , dissolution of bioclasts begins with fossils originally compos ed of aragonite and high magnesium calcite; in some cases, all calcite has been dissolved. The resultant moldic porosity is commonly enlarged by further dissolution of the rock matrix to form vuggy porosity. Both inter crystalline and vuggy types of porosi ty are commonly associated with dolomitized shelf facies . Thus, relatively early syngenetic dolomitiza tion can result in concordant sheets
Middle Shelf
331
.·;'
1
North
D 50 100 Miles LSZ!!�·I 7771
Figure 58-Paleogeog raphy of the Mid· die Cretaceous (Albian) Edwards Forma tion of Texas. Major structural and regional depos itional features and distribution of lithological types are in dicated (from Kerr, 1 977). Broad middle shelf environments are bounded on the southeast by the shelf-edge Stuart C i ty Reef Trend (afte r Pe rkins, 1 974).
Figure 59-Section of middle Glen Rose Limestone, Cretaceous (Albian) along Seco Creek, Medina County, Texas . Transgress ive·regressive sequence con sisting of four lithologic su bu n it s : (1 , 2) algal s t romatolite on r i ppled grainstone of inner shelf, (3) nodular-bedded, very fossi l ifero us normal marine bioclastic wackestone and calcareous shale of middle shelf, (4) Corbula beds and clay of inner shelf, and sol ution and col lapse zone of shelf interior (see also Fig . 60). ,
of porosity following intertidal facies . This dolomite may be followed by later discordant patterns of somewhat coarser dolomite which cuts across bedding and represents former water tables . Apparently, these formed through migrating lenses of meteoric water moving interstitial brines above or below them . Such dolomite may form completely uncontrolled by original sediment or grain permeabili ty and may overgrow dolomite form ed earlier . Its coarse grain-size may enhance permeability, but its tenden cy toward overgrowth and vein-filling (in addition to replacement of precur sor calcite) may destroy earlier porosity. The presence of fractures may com pletely control the economics of pro spects in dolomites with minor amounts of vuggy porosity. For ex-
ample, large Anadarko Basin gas fields in the Arbuckle Limestone and in the Hunton Group produce from fractured dolomites deposited originally as shelf facies with vuggy porosities of only 5 to l OOJo and inter crystalline porosities of but 3 to 6%. Chalky-textured limestones of shallow water origin may also be reservoirs, especially if fractured . Because the original carbonate sedi-
ment is highly porous , consideration to what processes effectively prevent cementation in buried shelf sediments should be taken. These include lack of burial below I ,000 m, absence of solution welding, and lack of a hydraulic head so that little water flows through the system. Further more, where early slight cementation has evenly rimmed calcarenite grains, the fringe of cement can prevent
332
Middle Shelf
FEET COR B U LA B E DS
Fossils incrude ech inoidsd!H Solenio,
V ERY FOSS ILIFE ROU S LIMESTONES AND
Hemiosler, ont/ Eno/loslerJ, (Poroc,slis), l a rge benthonic foraminifera � (Or/Jilo/ino), a n d a variety dosyclad a l g a e +
CALC A REOU S S H ALES
of moll uscs 4) 1 i n c l u d in g I o rg e 10
g a st ropod s (§) w a c k esto n e
I Ty!oslomo ) .
B u rr o wing c o m mo n � N o d u l a r beddin g .
developed a lgol lomanot col !lf intertidal env i ron m
Figure 60-Columnar section o f lower part of cliff section on Seco Creek, Glen Rose Formation, Medina County, Texas (see also Fig. 59).
solution-compaction welding after burial and the "locked-in fabric" can remain highly porous. This porosity, however, is commonly destroyed by a second, later generation of clear , blocky calcite. If brine rich in magnesium and low in calcium permeates carbonate muds and stagnates within the porous but not very permeable layers, it may prevent solution-compaction welding and the sediment retains the original high chalky porosity under some minimum of overburden. Shelf Carbonates as Source Rocks
Carbonate source rocks occur only in muddy carbonate facies. This ex cludes grainstones, packstones with a high grain content and low mud con tent, and most boundstones - a notable exception being the typically organic-rich algal stromatolites. Ap parently, this relationship is a func tion of preservation of organic material rather than of the original organic content . In modern lagoonal environments where carbonate mud is
accumulating today (Florida Bay and northeastern Yucatan), the content of organic carbon, much of it humic as well as algal , may range from 0.3 to 3 OJo . Sediment cores taken in Florida Bay show a thin, light colored oxida tion zone that extends down only a few inches from the sea floor. Beneath this, there exists a reducing zone of gray carbonate mud that yields a noticeable odor of hydrogen sulfide . In contrast, modern skeletal grainstones (for example, the Florida Reef Tract) do not display this ob vious reducing zone. This results from the higher grainstone permeabilities allowing greater ex change of surface waters through sediment . The degree of bioturbation may also affect the ability to preserve organic material in carbonates. Basinal carbonate facies are most likely the richest carbonate source rocks (the Permian Bone Springs Limestone of West Texas ; the Cogollo of the Maracaibo Basin; and the Jurassic Diyab-Darb of Saudi Arabia), although muddy limestones
of the inner and middle shelf may also be adequate source beds to charge nearby reservoir facies. In southern Florida's Sunniland Trend, inner to middle shelf mudstone facies of Cretaceous age are apparently source rocks for overlying reservoir facies. Data from Palacas ( 1 978) and Pontigo and others (1979) indicate an average of 0.550Jo organic carbon, 200 to 800 ppm of C 1 5 + hydrocarbons , and an algal origin for kerogen that is moderately mature. Total production from the Sunniland Trend amounts to approximately 55 million barrels of oil through 1977, the largest fields be ing Sunniland Field with an estimated ultimate recovery of 1 8. 8 million bar rels and West Sunoco-Felda Field with an estimated ultimate recovery of 50 million barrels of oil. In addi tion, Patacas (personal commun.) observed an even carbon number preference in the C20-C30 n-paraffins. This trend of even number preference may in time prove to be characteristic of carbonate source rocks; in contrast, clastic
Middle Shell
Figure 6 1 -Distribution
of
333
principal
facies In the lower Glen Rose reef inter· val
of central
Texas outc rops (after
Perkins, 1 974).
/
,/ II
I·
Oyster Biostromes
-a
�J�
Biostrome
Transition
NW
..,BP/W
Monopleurid Biostromes
DB
I�
I·
Figure 62-Cross-section of facies il· l u strated i n Figu re 6 1 , from the Llano uplift of central Texas to the southeast (offshore di recti on) (after Perkins, 1 974).
Caprinid Back reef
Caprlnid Mounds
DB
or
� P/W
Sea
Reefs JJa
+
Peri reef
Grainstone IIG
·I
·I SE Downdip Limit of Outcrop
25
20
15
10 !I
0
source rocks tend generally to display an odd-number preference. In general, carbonates have been discounted or underestimated as source rocks. However, it appears that carbonates, with their high con tent of non-calcified algal material, are more efficient hydrocarbon pro ducers than are clastics. Data from Gehman ( 1 962) and Claypool and Reed ( 1 976) indicate that the hydrocarbon content per unit of organic carbon is 3 to 4 times higher
in carbonates than in shales . This suggests that the lower limit cutoff of 1 Ofo organic carbon in traditional clastic source rocks would be about 0.25 to 0. 300fo for carbonates . The in creasing acceptance of fine-grained carbonates as source rocks was demonstrated oy a recent symposium on this topic entitled "Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks" (Geological Society of America 1 980 Annual Meeting, Atlanta) .
The problem often arises as to the ability of carbonate shelf facies to act as seals and to trap fluids or gas . The sheet-like geometry of shelf deposits and their basic ' 'layer cake stratigraphy" certainly encourage the formation of seals for reservoirs. The progradational pattern of the typical upward-shoaling cyclic sediments across shelves aids in sealing off mid dle shelf reservoirs with both up-dip and overlying fine-grained carbonates and shoreward sabkha evaporites.
334
Middle Shelf
s I
Tr a v is
D
I
Be l l
W i l l i a m son
LEGEND Argil!oceo!JS bio m1cnte WIth com rr·on ammon i tes
B i o c l a s t ic, i ntra c la s ti c , ool i tic
l i mestone
\
N M c Lennan
Hi l l
Johnson
Interbed ded limestone end dolomile Calcareous shale with common ommonites
Terrig enous sandstone
Figure 63-Cross section along
Fredericksburg outcrops f rom central to
north Texas i ndicating progradation of upwa rd shoal i n g facies in a northerly direction (f rom Rose, 1 972).
However , the brittleness of dolomite and limestone exceeds that of other sediment types so that carbonates tend to fracture more readily than terrigenous clastics . There is, then, no straightforward answer concerning the quality of sealing carbonates. Many carbonate rocks probably are of intermediate quality and leak fluids continuously. If the leaks are slow enough to permit the temporary accumulation of fluids, the beds are effective seals . The lack of strata with sealing capacity is the main weakness seen in evaluating some 3 ,000 m of Upper Cretaceous and Tertiary carbonates of south Florida and the Bahama Banks. Seawater invasion has occur red throughout this part of the sec tion. In a well on Long Island in the Bahamas , salt water extends down
past 5 ,500 m, as determined by a complete lack of any temperature gra dient to the total depth of the well (Meyerhoff and Hatten , 1 974). However, dead oil stains in these beds indicate the presence of migrated hydrocarbons high in the section. Predictive Stratigraphic Models of Shelf Sedimentation
Several models exist designed to predict the distribution of carbonate reservoir facies. In most carbonate sections, primary facies patterns con trol the distribution of porosity and permeability, and the description of carbonate shelves is carried out through extensive sample examination programs. Where diagenetic facies control the porosity and permeability, the overprint of diagenesis may be concordant with primary facies pat-
terns or discordant . An example of a concordant relationship occurs in the Jurassic Smackover Formation of southwestern Arkansas . At Walker Creek Field , Becher and Moore ( 1 976) found porous oolite grainstones only at high parts of oolite bars, parts subaerially exposed to meteoric diagenesis. The remainder of the bars were non-porous owing to calcite cement . In contrast, discordant relationships exist with regard to the distribution of dolomitic facies in the Hunton Group of the Anadarko basin where irregular facies boun daries cut across what must have been primary trends parallel to the strike of the basin (Amsden and Rowland, 1 967). Similar discordant trends of dolomitization are observable in the Madison of central and western Mon tana.
Middle Shelf
335
.---- ----- --· ----- ··-· · ··- ···· ··· ... ... ... . .... .. . .............. .
BEACH
R U D I ST PATCH R E E F -----.....__ a n d REEF DEBR IS
Figure 64-Deposltional
model
of the
Edwards Limestone {upper Fredericksburg and equivalent strata) in the area of Belton, Texas, showing p ro
gradational
inner and middle shelf facies
(Kerr, 1 977).
The steep shelf margin or drop-off model is the most reliable and useful in predicting facies. In such situa tions, the most favorable reservoir facies are commonly found at or near the shelf edge, but inner shelf dolomites are also prime reservoir facies . Patch reefs and shoals across the middle shelf are commonly pro ductive, but their distribution is usually non-systematic . On the other hand, the less regular facies developed on the ramp model are determined by the low ramp gradient . Very gentle sea floor inclinations of far less than I 0 occurred in broad epicontinental seas and intracratonic basins. The resultant facies pattern reflects the broad, subtle facies varia-
tions found in widespread shallow water environments (Irwin, 1 965 ; Wilson, 1 975). In contrast , steeper dips observed in the Smackover of the Gulf Coast demonstrate that nar row more distinct facies belts form with steeper ramp gradients. The shelf margin or drop-off model is well established and discussed in many papers. The basic facies belts of inner, middle, and outer shelf are generally recognized , and outer shelf reservoir facies are common explora tion targets . A real problem , however, is the lack of predictive depositional models within the middle shelf. Most of the shelf consists of irregular belts of facies and local patch reefs with no
trends . The only method for identify ing proximity to isolated organic buildups is through rather sophisticated ecological study of con tiguous biota, and this usually fur nishes only very local control. There are two regional stratigraphic situations across broad middle shelves where prediction of reservoir rocks is possible: ( 1 ) In stratigraphic traps in pro gradational sedimentary cycles (inner to middle shelf) . The normal spec trum of carbonate facies, seen in upward-shoaling cycles across shelves , places (following Walther's Law) muddy carbonate source rocks of middle shelf environments in lateral sequence next to high energy
336
Middle Shelf
Facies
Pore Type
Tidal and Supratidal Mud Flats
lnter ·Rhombic
Gastropods
Gastropeds
Skeletal
Moldic
Grainstones
Inter· Rhombic
Bafflestones
Rudist Ske l e tal
Moldic Q) Inter-Rhombic
Shallow Shelf Mobile Grain flats
Fig u re 55-Composite section of the Ed wards Limestone showing l it h ofa c i e s depositional environments, and p o r osity distribution. N ote shoaling upward, regressive sequence from middle to in ner shelf envi ronments (Kerr, 1 977). ,
South
North
K
Edwards A ense Zone Edwards 8
o�w
St
�L 10 k m
F i g u re 66-S helf to basin p r ofil e of t h e Edwards Limestone (total Fredericksb u rg Gro up) northwest to southeast from t h e S a n M a rcos Arch t o t h e Stuart City Reef Trend, after Griffiths and ot he rs ( 1 969), from Wilson (1 975). Dotted pattern i n dicates porosity d i str i but io n .
grainstones of the inner shelf, these in turn being adjacent to dolomitized in tertidal mudstones . The most shoreward facies is commonly im permeable sabkha anhydrite-gypsum which acts as an effective up-dip seal . The progradational sequence of the cycles generally results in basinward outstepping of the sabkha facies , placing the seal above, as well as up-
dip, t o excellent reservoirs. The Mississippian of the Williston Basin and Arab zones of the Jurassic across the Arabian Shield are classic ex amples. (2) Where regional disconformities across shelves produce important reservoirs. Leaching and dolomitiza tion of subjacent horizons can transform originally impermeable
Middle Shelf
��
�
�J!ore than 50% co.rbonatr;- is iime snnd .
_,:fit•__. Evaporite
p�rcP.r>toge of total cyd�
[.;:•li] More lh<J.n
I I
I
- Percentage anhydrite of 1otol cycle We lt _fl,Oi 1.§.] Thc\.kness of o.nhydrite
2S% corbonClte is time S\lnd.
line of dense brown lin1e mOJdslone facies Thi
� Prooi,ally all oot!JCX1ato: is d'..XIlmite.
- EdQe '
Figure 67-Facles of
Late Ju rassic Arab D zone ln Persian Gulf area (from
Wilson, 1 975).
micritic sediments into acceptable
zation plus fracturing have resulted in
Zones of Saudi Arabia, exists in
reservoirs. The top of the Edwards
sub-unconformity reservoirs.
coarse shoal grainstones and
Examples of Oil and Gas Production in Middle Shelf Carbonates
middle shelf facies . These were
over the San Marcos Arch is an ex
ample of such a situation. The wide spread Arbuckle-Ellenburger surface
at the top of the Lower Ordovician of
337
associated dolomitized wackestones of
As indicated above, the world's
the mid-continent is another instance
greatest single reservoir formation,
where complex phases of dolomiti-
the cyclic succession of Jurassic Arab
described in detail by Powers (1962) ,
followed by a regional facies descrip A vast shoal area lying between the interior of the
tion by Wilson ( 1975).
338
Middle Shell
�---
FAT E " -----� F"-1
F R- M
![
_ _ _
FR-L
B
Figure 68-Core analysis of Mishrif Limestone i n Fateh Field of D ubai off· shore i l l ustrati ng use of sonic log with gamma ray log for distinguishing porosi ty. Note permeability connected with coarse flanking beds of the near reef. After Jordan and others (1 981 ).
Arab craton and the Qatar-Dirang Namak High to the east (beneath the Qatar Peninsula and the Iranian coast) contains, in each of the six Arab and Jubaila cycles , middle shelf bioclastic grainstones, some oolite grainstones, and a minimum of mudstones and bioclastic wackestones . This area also separates the northern Basrah Trough and the southeastern Rub a! Khali Basin (Fig. 67). Giant anticlines such as Ghawar, Abqaiq, Manifa, Khurais, and the Dukhan cross this area. Wells are unusually productive in these Jurassic strata, being capable of initial oil pro duction of 8,000 to 1 2,000 b/d from coarse carbonate grainstones. Dukhan area on Qatar has less well-developed grainstones , with wells capable of half this production; farther east, into the Persian Gulf offshore, fields like Id el Shargi and Maydan Mazdam produce from Arab Zone dolomite partly of intertidal origin. These fields lie in an area of more restricted circulation over the Qatar-Dirang Namak High and away from middle shelf environments. It is noteworthy that although the great reservoir is produced by lack of cementation of sands produced in open shelf environ ment, the seal of each cycle is created by extensively prograded , restricted
K H AT I YA H MAUDDUD
marine and sabkha evaporites. Another great carbonate shelf exists along the Trucial coast bordering the Rub al Khali Basin on the north. The thick and relatively pure Thamama Limestone of early Cretaceous age stretches over the eastern Persian Gulf; the shelf area was tentatively outlined by Wilson ( 1 975). The se quence begins with a mostly pelagic lime mudstone (Habshan or Sulaiy Formations) both to the east and west of the Trucial Coast area, and develops upward through 300 m into a dolomitic shoal facies which is part ly intertidal. The overlying Lekhwair (Fig . 70) and Kharaib units represent a total of almost 500 m of alternating chalky, somewhat argillaceous, locally dense, stylolitic lime mudstones and wackestones with porous packstones and grainstones (Fig. 71). Porosity is both inter- and intragranular. Particle types include ( l ) algal and peloid types with numerous lumps and (2) worn bioclastic debris with certain large foraminifera. The dominance of codiacean green algae and some red algae indicates normal marine circula tion and the development of widespread shoals. There are about 20 such cycles across the shelf trending regularly from east to west within the Lekhwair
and Kharaib Formations (Hassan et a!, 1 975). The complex is capped by another cycle, one with more strongly differentiated facies, the Hawar Shale-Shuaiba Limestone. The Shuaiba forms a distinct east-to-west algal platform at the top of the Thamama with very well-developed, porous rudist facies. The major disconformity at the top of the Shuaiba and the localized influence of rising deep-seated salt domes resulted in meteoric alteration of the upper Thamama sediments. Oil and gas are present in great quantity across the middle of this platform in a broad north-to-south belt transecting the middle Trucial Coast . Hydrocarbons occur throughout the Thamama but are most abundant at the top of the Shuaiba and Kharaib Zone B. The source for this production is most probably the bituminous Hawar Shale (Murris, 1 979). Some of these Thamama fields, such as Bab, Bu Hasa, Id el Shargi , and Zakum are in the billion barrel range of ultimate production . Discoveries were made from 1 954 to 1 970. The geological examples of well studied middle shelf carbonates discussed above, the Late Paleozoic Leavenworth and Hueco Limestones , are typical shallow shelf deposits sur-
�J1trlrJip ShPI'
339
ALPAR RESOURCES INCORPORATED ffO. 1-10
SW/Sf SEC.
10
26N- SSf
STONtl MOUNTAIN SHAlf
Cak:areouo Shale
•o IIYtl
fOIMATION Non·Porous limeetone
Non�Porous
Limestone
Anhydrite
/
N�/·'
o om1te
...! ::: .. ..,
•, '
' t , .-·
...-:/:�--' ...,
G•m•o
log
0
10
Poron Sucroe•c Dolomite
lo,
Non·Porouo llmettone
100
••
20
10
•
-10
Figure 6 9 - U s e o1 neutron and electron dP.ns1ty logs :n m t e rprP.t ing poros1ty 1 n car bonat e strat a . Schl umberger corn· bination gamma ray, compensated neutron , and formation density log :rom
Alpar Resou rces Fed. 1 · 1 0 we l l .
Richland County, Montana; from the O r dovicia'l R e d R i v e r Formation. I l lustrates
the relationship o1 l ithology (c hecked by pe t r og r a phic analysis of cuttmgs) to ;og responses (from .A.squ i t h . 1 9 79)
rounding Earl y Pennsylvanian block faulted uplifts throughout the southwestern and mid-continent of the United States. Many fields pro duce from organic buildups in such carbonate strata. These porous and leached mounds include the Scurry County complex of north-central Texas , one of the largest fields i n the
pinnacles o ffshore in shallower Penn sylvanian basins . Pennsylvanian and Wol fcam pian shelves were most ly subject t o open circulation as in
Many fields have been st udied i n detail and several a r e reviewed b y Wilson ( 1 975) and G reenwood (in pres s ) . Greenwood est imates t h at
dicated by the common brachiopods,
Pennsylvanian strata in the
crinoids, coral s , and bryozoans fo und
southwestern U nited States should ulti mately produce 8 billion barrels of oil .
in benthonic limestones and shales. The reservoir rock i s of a more specialized biota , mainly of phylloid
continental United States, and hun
algae, encrusting tubular
dreds of smaller isolated fields along shelf margi n s , within the shel f, and as
Chaetetes.
foraminifera,
Tubiphytes, and
An excellent review of the Penn sylvanian shel f around the Paradox Basin is found in Bass and Sharps
( 1 963) : see Fi g u re 7 2 for the d i s t r i b u -
340
Middle Shelf
Figure 70-Thickness of Lekhwa i r For
mation, Thamama Group, early Cretaceous shelf facies across the Trucial Coast of eastern Arabia, United Arab E m i rates. Note. that the u nit doubles in thickness into the Rub al Kali basin (from Hassan , Mudd, and Twombley, 1 975).
Figure 7 1 -Lekhwair Formation of cross section west to east across United Arab Em i rate states showing multiple productive horizons i n early Cretaceous open shelf g rainstones and packstones (from Hassan , Mudd, and Twombley. 1 975).
Thamama Group;
Sl
Sl\uiJ,Iei1(ll
8ob l
I
Asob
East
1
---
"
·�
I.JI
Sul:ldlrision em C<>-rel>lion by e)dit: <290oimmt ..mts
Middle Shelf
Montrose
U TA H
341
Ill
lisbon
�
Paradox Basin
I I
B MontiLIIo I 1 I
Papoose " Canyon
COLORADO
lll Cortez
Black Mesa
•
Basin 0
0
10
16
20
Figure 72- Locaiion map and facies distribution in the Pa radox basin in Penn sylvanian time showing Aneth and White Mesa fields on Its southwestern shelf.
Miles
32 Km
�
A R I ZONA
tion of facies here. The cyclic Middle Pennsylvanian strata of this shelf are well exposed in the Monument Upli ft of southern Utah. East and north of this Laramide uplift, the buried por tion of the shelf surrounds the deeper Paradox evaporite basin and contains numerous small oil fields (White Mesa, Desert Creek and Ismay) and one large field, Aneth (Fig. 7 3 ) . Reservoirs are formed o f piles of phylloid algae with caps of grainstone (see also Fig. 43). Meteoric water has leached through the sediment , dissolv ing algal plates of probable aragonite composition and increasing the porosity and permeability .
The Viburnum Trend
Extensive lead-zinc deposits in Up per Cambrian carbonates of southeastern Missouri formed in oolite grainstones on the flanks of the Ozark Upli ft. The regional setting consists of a broad stable sandy area south of the Canadian Shield (Fig. 74), the classical Upper Cambrian (Croxian) area. To the south , these nearly littoral sands of the Canadian Shield grade into limestones and dolomites deposited on the flanks of the Ozark Uplift which rose episodically from early Cambrian through Pennsylvanian time (Thacker and Anderson , 1 977).
The Upper Cambrian section in Missouri is about 650 m thick and consists of a basal sandstone, the Lamotte, which is overlain by a thick sequence of mostly carbonate rocks. The oldest of these is the ore-bearing Bonneterre Form ati on In the Vibur num Trend, ore distribution is related to the geometry of an Upper Cam brian oolite belt about 30 mi long and 2 rni wide (Fig. 74). The Bonneterre facies here is a brecciated, dolomitiz ed oolite grainstone. Cross-bedding, exposure surfaces, and abundant in traclasts indicate frequent emergence of the oolite bar. To the east, in mid dle shelf environments behind the .
342
A
Middle Shelf
B-7
-¢-
B-1 •
A-14
-9-
CONOCO NovojoA-10 •
C-1 •
A-20 • B
;;
• .... 0
5!
Figure 7 3 C ros s section A-B west to east ac ross White Mesa Field, southern Aneth Field, Paradox basin , New Mexico. -
bar, burrowed, dolomitized lime mudstones and planar algal stromatolites were deposited in a very shallow, backbar lagoon (Howe, 1 968). Inner shelf sediments lapped up onto the Ozark Uplift which had considerable relief at the time. In front of the oolite bar, lime mudstones, wackestones, and green shales were deposited. The whole system can be con sidered a "shelf within a shelf. " Thus, the basic elements of inner, middle, and outer shelf environments are present at different scales. At the larger scale, there is a progression from inner shelf sandstone facies near the Canadian S hield to mixed sandstone-limestone facies of the mid dle shelf in the mid-continent area . Around the Ozark Uplift , similar facies developed on a shelfbuilt out ward from the Precambrian granite core of the uplift. Thus, the Bon neterre oolite bar is a local outer shelf deposit, but in the larger sense is a middle shelf grainstone shoal, formed on the flanks of a local high. SELECTED REFERENCES Amsden, T. W., and T. L. Rowland, 1 967, Silurian-Devonian relationship in Oklahoma, in International symposium on the Devonian system, Vol. 1 : Calgary, Alberta Soc. Petroleum Geols. Asquith, G. B., 1979, Subsurface carbon ate depositional models; a concise
review: Tulsa, Petroleum Pub. Co., 1 2 1 p. Ball, M . M . , 1967, Carbonate sand bodies of F1orida and the Bahamas: Jour. Sed. Petrology, v. 37, p. 556-59 1 . Bass, R . 0 . , and S . L . Sharps , eds . , 1963, Shelf carbonates of the Paradox Basin: 4th Field Conf. , Four Corners Geol . Soc . , 273 p. Beach, D. K . , and R. N. Ginsburg, 1980, Facies succession, Plio-Pleistocene car bonates, northwestern Great Bahama Bank: AAPG Bull . , v. 64, p . 1 634- 1 642. Becher, J. W . , and C. H. Moore, 1 976, The Walker Creek Field, a Smackover diagenetic trap: Trans . , Gulf Coast Assoc. Geol. Socs., v. 26, p. 34-56. Claypool, G. E . , and P . R . Reed, 1976, Thermal-analysis technique for source rock evaluation; quantitative estimate of organic richness and effects of lithologic variation: AAPG Bull. , v. 60, p. 608-612. Dunham, R. J., 1970, Stratigraphic reefs versus ecologic reefs: AAPG Bull . , v . 54, p. 193 1 - 1 932. Enos, Paul, 1982, Restricted shelves , bays, lagoons , in P. Scholle, D. Bebout, and C. Moore, eds . , Carbonate depositional environments : AAPG Mem. 33, this volume. Fisher, W. L . , and P. U. Rodda, 1 969, Edwards Formation (Lower Cretaceous), Texas ; dolomitization in a carbonate platform system: AAPG Bull . , v. 53, p. 55-72. Gehman , H . M. , Jr. , 1 962, Organic mat ter in limestones : Geochim. et Cosmochim. Acta, v. 26, p . 885-897. Ginsburg, R . N . , R. Rezak, and J. L. Wray, 1 972, Geology of c alcareous
algae: Univ . of Miami , Miami, Fla . , Sedimenta 1 , Comp. Sed. Lab . , 40 p . Greenwood, E . , in press, Pennsylvanian production in southwest U.S.A. : Ur bana, Ill . , Proc. , 9th Intern . Car boniferous Cong. Griffith, L. S., M. G. Pitcher, and G. W. Rice, 1 969, Quantitative environmental analysis of a Lower Cretaceous reef complex, in G. M . Friedman , ed., Depositional environments in carbonate rocks: SEPM Spec. Pub. 14, p . 120- 1 38. Harris , L . D., 1973, Dolomitization model for Upper Cambrian and Lower Or dovician carbonate rocks in the eastern United States: Jour. Research, U . S . Geol. Survey, v. 1 , p. 63-78. Hassan, T. H . , G. C. Mudd, and B. N. Twombley, 1975, The stratigraphy and sedimentation of the Thamama Group (Lower Cretaceous) of Abu Dhabi: Dubai, 9th Arab Petroleum Cong . , v. 107 (B-3) p. 1 - 1 1 . Hine, A . C ; , and A . C . Neumann, 1977, Shallow carbonate bank margin growth and structure, Little Bahama Bank, Bahamas : AAPG Bull . , v. 61, p. 376-406. Houbolt, J. J. H. C . , 1 957, Surface sed iments of the Persian Gulf near the Qatar Peninsula: Utrecht , Netherlands , Univ . Utrecht, Master's thesis, 1 1 3 p . Howe; W. B . , 1 968, Planar stromatolite and burrowed carbonate mud facies in Cambrian strata of the St. Francois Mountain area: Mo. Geol . Survey and Water Resources, Rept . Inv. 4 1 , 1 1 3 p. Irwin, M . L . , 1965, General theory of epeiric clear water sedimentation: AAPG Bull. , v. 49, p. 445-459. Jordan , C. F. , Jr., 1973, Carbonate facies
Middle Shelf
343
m udstone ond shale facies ·
Figure 74-Facies distr ibution of the Bonneterre Formation (Upper Cambrian), southeast Missouri Lead M i n ing District; note the position of the Old Lead Belt and the newer Viburnum Trend in the "outer shelf" environments which form ed on the flank of the Ozark uplift as a sma l l shelf system with i n a larger shelf (the stable mid-continent area). Black areas m a rk Precambrian outcrops.
and sedimentation of patch reefs off Bermuda: AAPG Bull . , v. 57, p . 42-54. __ 1975, Lower Permian (Wolfcamp ian) sedimentation in the Orogrande Basin, New Mexico : 26th Field Conf. Guidebook, New Mexico Geol. Soc . , p . 109- 1 1 7 . __ and J . L . Wilson, 1 97 1 , The Late Paleozoic section of the Franklin Mountains: Field Conf. Guidebook , Permian Basin Sec . , SEPM, p . 77-86. __ T. C. Connally, Jr . , and H. A. Vest, in press, Upper Cretaceous car bonates of the Mishrif Formation, Fateh Field, Dubai, U.A. E . , in P. 0. Roehl and P. W . Choquette, eds . , Car bonate petroleum reservoirs: New York, Springer-Verlag Pub. Kerr , R. S . , 1977, Development and dia genesis of a Lower Cretaceous bank complex, Edwards Limestone, north central Texas , in D. Bebout and L . Loucks, eds . , Cretaceous carbonates of Texas and Mexico: Austin, Tex., Bureau Econ . Geology, p . 2 1 6-233. Meyerhoff, A. A., and C. W. Hatten, 1 974, Bahamas salient of North America; tectonic framework , stratigraphy, and petroleum potential: AAPG Bull . , v . 58, p. 1 20 1 - 1 239. Murris, R. J . , 1 979, Hydrocarbon habitat of the Middle East, in A. D. Miall , ed . , Facts and principles o f world petroleum occurrence: Can . Soc. Petroleum ,
Geol s . , Mem. 6, p. 765-800. Palacas, J. G . , 1978, Preliminary assess ment of organic carbon content and petroleum source rock potential of Cretaceous and Lower Tertiary car bonates, South Florida Basin : Trans . , Gulf Coast Assoc. Geol. Socs. , v. 28, p . 357-38 1 . Perkins, B . F . , 1974, Paleoecology of a rudist reef complex in the Comanche Cretaceous Glen Rose Limestone of central Texas, in Geoscience and man, v. 8, Aspects of Trinity Division Geology: Baton Rouge, La. State Univ . School Science, p. 1 3 1 - 1 73 . Pontigo, F. A . , Jr. , e t al, 1 979, South Florida's Sunniland oil potential: Oil and Gas Jour. , v. 77, p. 226-232. Powers, R . W . , 1 962, Arabian Upper Jurassic carbonate reservoir rocks, in W . E . Ham, ed . , Classification of car bonate rocks , a symposium: AAPG Mem. 1 , p. 1 22- 1 92. Purdy, E . G., 1 96 1 , Bahamian oolite shoals: AAPG Bull . , v. 45, p. 53-62. __ 1974, Karst-determined facies pat terns in British Honduras; Holocene carbonate sedimentation model : AAPG Bull . , v. 58, p. 825-855. Rose, P . R . , 1972, Edwards Group, sur face and subsurface, central Texas : Austin, Univ. of Texas , Bur. Econ . Geol . , Rept. Invest . 74, 198 p . Shaw, A. B . , 1 964, Time i n stratigraphy:
New York, McGraw-Hill Pub . , 353 p. Thacker, J. L . , and K. H . Anderson, 1977, The geologic setting of the southeast Missouri lead district-regional geologic history, structure, and stratigraphy: Econ. Geology, v. 72, p . 339-348. Toomey, D. F., 1 969a, The biota of the Pennsylvanian (Virgilian) Leavenworth Limestone, mid-continent region , part I; stratigraphy, paleogeography, and sediment facies relationships: Jour. Paleontology, v. 43, p. 1001- 1 018. __ 1 969b, The biota of the Pennsyl vanian (Virgilian) Leavenworth Limestone, mid-continent region , part II; distribution of algae: Jour. Paleon tology, v. 43, p. 1 3 1 3 - 1 330. __ 1 972, The biota of the Pennsyl vanian (Virgilian) Leavenworth Limestone, mid-continent region, part III; distribution of calcareous foraminifera: Jour. Paleontology, v. 46, p. 276-298. Wilson, J. L . , 1 970, Depositional facies across carbonate shelf margins: Trans . , Gulf Coast Assoc. Geol. Socs . , v . 20, p. 229-233 . __ 1 974, Characteristics o f carbonate platform margins: AAPG Bull . , v. 58, p. 8 1 0-824. __ 1975, Carbonate facies in geologic history: New York, Springer-Verlag Pub. 439 p .
346
Reef
Figure 1 -A sketch il lustrating the major facies of a fossil reef (afte r James, 1 979, with perm ission of Geol. Assoc . Can ada ).
BAFFLESTON E
BIN DSTO N E
FRAM ESTO N E
Figure 2-An interp retative sketch of the di fferent types of reef l imestone recogn ized by Embry and Klovan (1 97 1 ) (afte r James, 1 979, with permission of Geol . Assoc. Canada).
A
reef, rising above the sea floor, is an entity of its own making - a sedimen tary system within itself. Numerous, large calcium carbonate secreting organisms stand on the re mains of their ancestors and are sur rounded and often buried by the skeletal remains of the many small organisms that once lived on, beneath and between them. Because they are built by organisms, fossil reefs are storehouses of paleontological information and modern reefs are natural laboratories for the study of benthic marine ecology. Furthermore, fossil reefs buried in the subsurface contain a disproportionately large amount of our oil and gas reserves compared to other types of sedimentary deposits. These two facts alone have resulted in reefs being studied in more detail by paleontologists and sedimentologists , than perhaps any other type of sedimentary deposit. Consequently,
FLOATSTO N E
there is a large and diverse source of data for the student of reefs to draw upon when studying these deposits. Unlike other sedimentary deposits, a reef is not entirely the product of physical sedimentation but is almost entirely the physical expression of a community of organisms, growing in one place for an extended period of time. These communities have, however , changed dramatically through the course of geologic time, so that a reef at any one time may be quite different from a reef only a few million years younger or older .
R U DSTO N E
Because o f this, any synthesis of reefs must be largely comparative; not only a comparison of modern and ancient reefs , but a comparison of different fossil reefs as well . With this guiding tenant I have, in the following pages , outlined the attributes of different types of modern reefs. These ex amples are from the Atlantic Caribbean area, the region with which I am most familiar. This sec tion is followed by a discussion of the terminology used in describing an cient reefs, and then by a synthesis of the characteristics of these fossil
Reef
ATTACH E D S E G M E NTED CALCAR E O U S B E NTHOS
CAV IT I E S WITH I NT E R NA L S E D I MENT
E N C R U ST I N G M ETAZOA
347
DOMAL & MASS IVE M ETAZOA
RASPI N G F I S H SEDIMENT
Fi g ure 3-A sketch illustratin g the dif ferent aspects of the organism/sedi· ment mosaic that is a reef (after James, 1 979, with permission of Geo l . Assor;; . Canada).
structures . To illustrate the pertinent textures and fabrics of those car bonates, the text is supplemented by examples of modern and ancient reef carbonates . CLASSIFICATION OF REEF LIMESTONES
Before discussing the deposits themselves there should be some con sensus as to the classification of dif ferent types of reefal carbonates. Because reef rocks are often compos ed of large components and are com monly formed in place by the growth of organisms and therefore not physically deposited, they are not amenable to the type of classification schemes outlined by Folk ( 1 962) and Dunham ( 1 962) . Folk recognized reef limestones as different and called them biolithites, recognizing that this was a general term and that these rocks needed to be subdivided. Dunham, on the other hand, sug-
gested the term boundstone for those reef rocks showing signs of being bound during deposition. The most descriptive and widely ac cepted scheme, however, is a modification of Dunham's ( 1 962) classification of carbonate sand and mudrocks proposed by Embry and Klovan ( 1 97 1 ) at the University of Calgary (Fig. 2). They recognize two kinds of reef limestone, alloch thonous and autochthonous. The allochthonous limestone is the same as the finer grained sediments, but with two categories added to encom pass large particles. If more than l OO!o of the particles in the rock are larger than 2 mm and they are matrix supported, it is a floatstone; if the rock is clast supported it is a rudstone. The autochthonous lime stone is more interpretative; frame stones contain in-place, large fossils that formed the supporting framework; bindstones contain in place, tabular or lamellar fossils that encrusted or bound the sediment to gether during deposition; and baffle stones contain in-place, stalked fossils that trapped sediment by baffling .
MODERN REEFS
Reefs in the modern ocean are commonly envisaged in terms of the prolific, shallow water , coral reefs of the Caribbean and Indo-Pacific. In the geologic record, there is a much wider concept of reef or carbonate build-up, and, under the heading of Modern Reefs, we must include not only the familiar coral reefs , but also most other in-place accumulations of skeletal calcium carbonate that rise above the sea floor. This wider definition includes other shallow reefs composed almost entirely of algae, banks of branching coral and calcareous algae, banks of mostly lime mud and build-ups of coral, and skeletal sediment in deep water. Dynamics of Reef Sedimentation
The present state of any reef is a delicate balance between the upward growth of large skeletal metazoans, the continuing destruction of these same organisms by a host of rasping, boring and grazing organisms, and the prolific sediment production by rapidly growing, short-lived, attached
348
Reef
Fig u re 4-A sketch of two coral heads and some of the organisms responsible for bioerosion (after Ginsburg and James, 1 974, with permission of the
University-of M iami).
calcareous benthos (Fig. 3) . The large skeletal metazoans (cor als) generally remain in place after death , except when they are so weakened by bioeroders that they are toppled by storms. The irregular shape and growth habits of these reef-builders results in the formation of roofed-over cavities inside the reef that may be inhabited by smaller, at tached calcareous benthos, and may be partly to completely filled with fine-grained "internal" sediment. En crusting organisms grow over dead surfaces and aid in stabilizing the structure. Branching reef-builders fre quently remain in place, but just as commonly are fragmented into skeletal conglomerates around the reef. Most reef sediment is produced by the post-mortem disintegration of organisms that are segmented (for ex ample, crinoids, in the Paleozoic and Mesozoic , calcareous green algae in the Cenozoic) or non-segmented (bivalves, brachiopods, foraminifers , etc), that grow i n the many nooks and crannies between the larger skeletal metazoa. The remainder of the sediment is produced by the various taxa that erode the reef - boring organisms (worms, sponges, bivalves) producing lime mud; or rasping organisms that graze
the surface of the reef (echinoids, fish) producing copious quantities of carbonate sand and silt (Fig. 4) . These sediments are deposited around the reefs as an apron and also filter into the growth cavities to form inter nal sediment, which is characteristically geopetal . Coral-Algal Reefs
The most widespread and volumetrically important reefs in modern oceans are constructed by hermatypic corals and calcareous algae. They are best developed and most successful on the windward sides of shelves , islands and platforms where wind and swell are consistent and onshore. The asymmetry of many ancient reefs and distribution of sedi ment facies suggests that this was so in the past as well. The reason for the preferential development of reefs on the windward side is by no means established, but sedimentation is like ly the most important control. Shallow water reef-building species characteristically produce abundant fine sediment, yet the maj or reef builders, because they are filter feeders and micropredators are in tolerant of fine sediment . The open ocean, windward locations are the on ly ones in which fine sediment is con tinuously swept away. Growth of reefs irito this zone of onshore waves and swell forms a natural breakwater of linear barrier reefs . If the shelf is narrow, this structure is often hard up against the
shoreline and is therefore called a fringing reef. Because the open-ocean swells and storm waves are absorbed by this barrier, the shelf and lagoon behind is commonly a relatively quiet environment, with the water stirred up only by wind waves, tidal cur rents, and occasional cyclonic storms. Here reefs are generally isolated structures, called patch reefs or lagoon reefs . Patch Reefs
These reefs (Figs. 6 to 1 3) have been particularly well documented in Bermuda, and the following descrip tion is summarized from Garrett and others ( 1 971). The smallest elements of the Ber muda patch reefs are coral knobs (Fig. 5); they are intergrowths of cor als, algae, and associated organisms that may be up to 5 m across and rise up I to 3 m above the surrounding sand-floored interareas . Larger reefs are for the most part aggregates of coral knobs. These reefs are generally unzoned , rising from depths as much as 20 m to within a few meters of mean low water. The patch reefs range in shape from pinnacles, to walls, to micro-atolls, which enclose a small sandfloo red lagoon. Corals cover from 10 to 45o/o of the hard substrate and comprise 40 to 80% of the solid reef mass. The bulk of most reefs is made up of domal or massive corals (Montastraea and Diploria) as revealed by dynamited sections . Coral knobs on the reef tops
Reef
Figure 5-A aariEIS or Sl<etcM;;; il· lustrating the different types or coral· algal ree fs on the Bermuda Platform {from Ginsbu rg antj Schroe
349
350
Reef
Figure 6-An areal view of numerous coral patch reefs on the Belize shelf ris· ing from depths of about 5 m. The dark brown areas are living coral and the buff colored areas are skeletal sand. The off. reef areas a re muddy carbonate sands colonized by meadows of Thalassia grass. The two reefs at the lower left are about 30 m in length.
Fig u re 7-An areal view of a complex of rhomboid-shaped platform reefs on the B�elize shelf. The complex in the foreground is about 6 km in length. These structures rise from water more than 30 m deep and their steep (more than 30°) sides support a wide variety of corals. Their shallow margins are either zoned like other reefs or a community of delicate branching corals and algae while the centers are mostly m uddy skeletal sands.
are dominantly the corals Mon tastraea, Diploria and Porites astreoides. Millepora, an encrusting and bladed calcareous hydrozoan, is to be found on the reef face and reef top where it encrusts flat surfaces or the branched stubs of dead sea fans and sea ships. Branching corals such as Oculina and Madracis decactis grow near the base of the reef face. The thin saucer-shaped coral Agaricia fragilis occurs only beneath or behind overhanging proj ections. Bun-shaped Siderastraea and
Isophyllia grow near the bases of cor al knobs . Most of the rock surface is covered by coralline algae which also encrusts dead coral and skeletal rubble on the reef top. Other encrusting organisms such as a few ectoproct bryozoa and bivalves (Chama, Pseudochama, and Spondylus) are also common . Flabelleform algae and soft corals bush out over much of the reef rock surface. The two most common algae, Sargassum and Dictyota, are geologically unimportant as they have
no calcareous hard parts. However, the shrub-like calcareous alga Stypopodium, Galaxaura, Padina, Udotea, and Neomeris produce fine sediment, while the branching forms Goniolithon, Amphiroa, and especial ly Halimeda produce abundant sand sized sediment . The alcyonarian or "soft" corals Gorgonia and Plex aurella also produce fine sand-size calcareous spicules. Boring organisms can be found in every piece of rock broken from the reef. Predominant among these are
Reef
351
Figure 8-A truck ti r e in 5 em of water i n Eniwetok lagoon on which corals (Acropora) have settled and begun to grow. This tire has been in place less than 20 ye a rs (photo by R. Slater and J . Warme).
Figu re 9-A coral knob on top of a small patch reef near Gou l di ng Cay, Bahamas, illustrating a di ve rse coral community composed ·of Diploria (large dome about 1 m across at center), Porites astreoides (ye l low coral at right and left), Porites porites (wh ite bra nching coral at center), Agaricia (leaf-like c ora l at lower left) and Acropora palmata (top left). The da rk branching organism at the top center left is an octocoral which has no skeleton, but only s m a l l calcareous spicules embedded in the soft tissue.
the rock-boring clams, (Lithophaga, Nigra, and Speng/aria rostrada), several species of polychaete worms , endolithic algae, and boring sponges (Cliona and Siphonodictyon). The
spiny echinoderm Diadema anti/larum grazes the reef surface while the par rot fishes (Sparisoma virde and S. abi/lgardi) graze the surface flora, in gest sand from the valleys, and occa sionally take bites of hard coral and coralline algae. The interiors of these reefs contain many cavities, inside corals, from the
boring by organisms and from the iregular overgrowths of encrusting organisms. The larger cavities (growth cavities) are due to biological roofing over , chiefly by corals. The catacomb of chimneys, tunnels and cavities in side the reef contains a distinctive community of organisms . Near the outside, cavity walls are encrusted with the right valve of the bivalve Spondylus americanus, coralline algae, ectoprocts, serpulid tubes, and the red foraminifer Homotrema rubrum. Deep within the dark
recesses of the reef, the proportion of the wall covered by living organisms decreases to zero . Garrett and others (197 1 ) have estimated that from 30 to 50% of the reef volume is occupied by open cavities and sediment fill. Sediments on top of the reef are mainly coarse to very coarse gravels and sands composed of whole to fragmented corals, coralline algae, the attached foraminifer Homotrema, the segmented calcareous alga Halimeda and bivalves. Every piece of the larger particles is bored by endolithic
352
Reef
Figure 1 0-T h e top of a patch reef near An dros Island, Bahamas, composed almost entirely of small, i r regular Porites astreoides colonies. The white areas are dead coral enc rusted with coralline algae while the dark areas in shadow are entrances to an extensive network of g rowth cavities inside the reef.
Figure 1 1 -The crest of a small coral knob on top of a patc h reef In 1 m of water on the Belize shelf. The large multllobate colonies are Montastraea an nularis (c om pare the shape of this col ony to the g rowth form of one in deeper water illustrated in Figs. 22 and 23) wh ile the smaller finger-like colon ies are Porites porites. The areas of g reen be tween corals are lush g rowths of the segmented, platey, calcareous algae Halimeda (p h oto P. Scholle).
algae and encrusted by coralline algae, ectoproct bryozoa, and Homotrema. Some of the reef sands are washed down the reef face to build a steep sediment slope which grades outward into lagoon sediments; at a distance of 1 0 m or more from the reef, reef-derived con stituents become quantitatively unim portant . The grain-size of the sedi ment decreases rapidly down the sedi ment slope and out onto the lagoon
floor. The fine grained sediments either filter down into the reef cavities where the amount of fine sediment is considerable (as evidenced when blasting open the structures) or it is winnowed, settling out of suspen sion as a halo of fines around the reef. While the distribution of different corals and calcareous algae is more or less uniform over the patch reefs of the Bermuda platform , in other areas
the structures are distinctly zoned . Patch reefs in the lagoons and shelves of the Belize barrier and atoll reef complex illustrate this well (Fig . 1 3) . I n some areas, the zonation i s simply one of more abundant coral growth on one side (windward) than the other (Shinn et al, 1 977). In others, there is a pronounced assymetry to the structures, and the corals are distinctly zoned. In most cases the limestone formed
Reef
353
Figure 1 2_;_The wall·l ike margin of a low relief patch reef on the Belize shelf. The 2 m deep channel, floored by skeletal sand, is bounded by n u merous multi lobate Montastraea annularis col onies (photo P. Scholle).
Figure 1 3-A series of sketches Il lust rating the different profiles of and principal corals on coral-algal patch reefs In Glovers Atoll lagoon, Belize (data from Wal lace and Schafe rsman, 1 977, and personal observation).
+- WAVES CORALS
� .. ..
Massive branching A. palmata Delicate branching A. cervicornis Massive, domal Dlplorla, Montastraea Slderastrea
:
. .
354
Reef
RUDSTO N E GRAI NSTONE B I NDSTO NE
BAFFLESTON E FLOATSTO N E
-t
- BACK R E E F
BAFFLEST O N E
REEF CR EST
B I NDSTONE
FRAM ESTONE
GRAINST O N E RUDSTONE
-+
REEF ---��- R E E F FRONT FORE R EEF FLAT .... Wa ves & Swell
Fig ure 1 4-A cross-section through a zoned marginal reef i l l ustrating the d if ferent reef zones, spectrum of different l i m estones produced i n each zone, and e nvironment of different reef-building organisms. In many modern reefs, the reef c rest is occupied by the massive branching coral Acropora palmata, a situation t h a t is rare in the fossil record (after James, 1 979, with permission of Geol. Assoc. Canada).
F i g u re 1 5-An a real view loo k in g north a l o n g the Belize ba r r i e r reef o n a calm day. The reef crest is the sharp dark line curving toward the upper right with dee p water to the lower right. The wide, light· g reen area shelfward of the very narrow reef crest is a 1 to 2 km wide blan ket of skeletal sand (mostly Halimeda) with zones of co ral r u b b l e in the lee of th e reef crest. Passes th ro ug h the barrie r (upper right) are a s m u c h a s 2 0 m deep. Numerous patch reefs can be seen on the shelf behind (upper left). (Photo R. N. Ginsbu rg ) .
in the patch reef is largely coral framestone to coral bindstone with obvious cavities filled with geopetal skeletal wackestone to sometimes packstone. The reefs are surrounded by skeletal, mainly green and red calcareous algae, foraminifer , and coral rudstones and grainstones. In
the rock record , crinoid skeletal grains commonly take the place of the calcareous algae. These grade out ward into progressively more muddy and well-bedded sediments. Platform Margin Reefs
Reefs along the platform edge (Fig .
14) range from almost continuous barriers to isolated patches of coral, irregularly distributed along the margin . In some areas , such as Ber muda, the platform margin is in water 5 m or more deep and the sea floor is covered by wide areas of cor al growth (the ledge flats in Fig. 5),
Reef
355
METE:RS
r.: Fig u re 1 6-An isometric d iag ram il· lustrating the main morphological elements of the shallow Belize barrier reef margin (modified from James et a l , 1 976).
Figure 1 7-An area l view of the spur and groove zone off Sand Key, Florida. The re ef flat is at the top and deep water at the bottom. Individual spurs are 5 to 1 0 m across (photo E. A. Shinn).
Figure 1 8,.--A forest of massive branch· ing Acropora palm ata on the crest of a patch reef on the Bel ize shelf. Individual branches are about 20 em i n diameter (photo P. Scholle).
356
Reef
Figure 1 9-The transition between two zones of reef g rowth on the reef front off Discovery Bay, Jamaica; the shallow zone to the right is dominated by Acropora palma ta growth and the deeper zone i n water depths of about 7 m to the left exhibits a wide variety of corals. The rounded multilobate coral colonies in the foreground are Mon tastraea annularis.
Figure 20-A diverse coral community in the lee of the reef crest behind Carysfort Reef, Florida, in water 2 to 3 m deep. The three most obvious corals are Acropora palmata (large branching form), Acropora cervicornis (delicate branching form), and Montastraea an nularis (massive m ultilobate form).
mainly massive and hemispherical col onies with only algal cup reefs break ing the surface. More commonly, the platform margin reefs, as seen from the air, are true barrier reefs, well zoned complexes of living reefs and the sediments derived from them (Fig. 15). The shallowest part of the com plex is the reef crest , growing in the
zone of breaking waves. Most of the coral-algal reef is found seaward of the reef crest, growing on an ir regularly sloping ramp called the reef front . Seaward and basinward of the zone of coral and algal growth is the fore reef, an area of reef debris, car bonate sand, blocks of limestone and coral fragments, as well as a variety
of different benthic organisms. In the lee of the reef crest is a very shallow zone that is often awash at low tide, called the reef flat . This style of plat form margin complex is often segmented by channels bisecting the reef. Skeletal sands are sometimes funneled basinward through these channels.
Reef
357
Figu re 21 -The margin of a deep-water m) coral spur covered almost enti re ly by a n intergrowth of Acropora cer vicornis, off Discove ry Bay, Jamaica. The coral sticks here are longer and much more delicate than in shal low water. The blue color is caused by the low light intensity and absorbtion of all but the blue end of the light s pe ct r um by the ove rlying wate r column .
(45
Figure 22-Looking down on a large col ony of Montastraea annu/aris composed of numerous overlapping plates of coral at a depth of 55 m off Discovery Bay, Jamaica (hammer on top of the colony at right center for scale).
Figure 23-The margin of a large colony of plate-like Montastraea ann u/ar/s at a depth of 55 m off Discovery Bay, Jamaica. The whispy growths in the left foreground are octocorals (diver for scale).
358
Reef
Figure 24-A small stand of stubby Acropora cervicornis about 1 m high in
the lagoon of Eniwetok Atoll at a depth of 4 m (photo J. Warme).
Figure 25-A close view of the su rface of a seaward-facing spur at a depth of 7 m on the Florida reef tract. The bladed colonies are the hydrozoan Mi/lepora and between these colonies are scat tered Porites astreoides colonies. The black, splney organism Is the echinoid Diadema antillarum which continuously scrapes the su rface of the reef rock grazing on algae and eroding the coral. The white areas are reef rock partially coated with coralline algae.
Figure 26-A huge single colony of Mon tastraea annularis at a depth of 20 m on the Flower Gardens Bank in the Gulf of Mexico off Texas (photo E. A. Shinn).
Reef
359
Figure 27-A slabbed and polished piece of reef limestone from a shallow coral spu r in 2 m of water on the eastern ocean-facing reef front of Glovers Atoll, Belize. The rock i s com posed of branches of the hyd rozo an Miflepora (center) which are encrusted by the red foraminifer Homotrema rubrum. The sediment between the branches is a submarine cemented ske l eta l g rainstone (see thin section photograph in Fig. 29). The holes at t h e edge of the sample are excavated by boring sponges, with holes cutting both skeletal elements and cemented sedi ment.
Figure"-28-A thin section u nder plane
polarized l ig h t of cemented reef l imestone from shallow Belize reefs. The section is stained with Clayton Yellow so
all aragonite components are white and all Mg-calcite components are red or pink. The rock is a packstone with sand· size grains of coral (aragonite, upper left) in a matrix of fine sand and sil t si ze peloids. The peloids are Mg-calcite micrite and the cement around the peloids is Mg ca lci te micrite and bladed spar. Scale bar 200 microns. -
-
Reef Crest Zone
This is the highest part of the reef at any stage in its growth , and if in shallow water, it is that part of the reef top that receives most of the wind and wave energy. The composi tion of the reef crest depends on the degree of wind strength and swell. In areas where wind and swell are in tense, only those organisms that can encrust , generally in sheet-like forms, are able to survive. When wave and swell intensity are only moderate to
strong , encrusting forms still dominate but are commonly also bladed or possess short, stubby bran ches . In localities where wave energy is moderate, hemispherical to massive forms occur with scattered clumps of branching reef-builders, although the community is still of low diversity. The lithologies formed in these three cases would range from bindstones to framestones .
Reef Front Zone
This zone extends from the surf zone to an indeterminate depth, generally no deeper than 1 00 m . On modern reefs there is commonly a steep cliff at this point whereas in fossil reefs, the zone of abundant skeletal growth grades into sediments of the forereef zone . Direct com parison between modern reefs and an cient reefs is difficult because today the sea floor from the surf zone to a depth of 12 m or so is often
360
Reef
F i gu re 29-A t h i n section under plane polarized light and stained with C layton ye llow of the grain ston e illust rated in Figu re 27. Th e angular aragonite grains are coral while the M g c al c i te grains are branching coral l ine algae, benthic foraminifers and echinoid particles. The cement is i sopachous rinds of M g calc i te bladed spar. Scale bar 1 00 microns. ·
-
Figure 30-A sample of reef wall l imestone from the Belize barrier reef (ci rca 1 3,000 years old). In t h i s example a cavity (the walls of which can just be seen at upper right and upper left) is almost comp letely filled with spherul itic g rowths of botryoidal aragonite. The open space remaining in the center of t h e void i s floored with a thin layer of mar i n e sediment which partially buries some of the aragonite (sc al e In em).
dominated by the robust branching form Acropora palmata, a species which developed only recently, in the late Pleistocene. Such branching forms are rarely found in ancient reefs. Instead, the most abundant forms are massive , laminar to hemispherical skeletons, forming framestones and sometimes bind stones. This zone commonly has a distinct
topography called the "spur and groove' ' consisting of a series of linear reef ridges running seaward and separated from one another by sedi ment floored channels. This is best developed in shallow water or at breaks in slope on the ramp. The main part of this zone sup ports a diverse fauna with reef builders ranging in shape from hemispherical to branching to colum-
nar to dendroid to sheet-like. Ac cessory organisms and various niche dwellers such as brachiopods, bivalves , coralline algae, and green segmented calcareous algae (Halimeda), are common. On m odern reefs where the reef-builders are cor als , this zone commonly extends to a depth of 30 m or so. The most com mon rock type formed in this zone would still be framestone but the
Reef
361
Figure 3 1 -A thin section of Belize reef wal l limestone (circa 9,000 years old) under plane polarized light in which cor· al has been bored by a bivalve and the boring has been filled by a complex suite of marine sediments and ce m ents. The section is stained with Clayton Yellow so that all aragonite components are white or light g ray and all Mg·calcite com· ponents are red or pink. The cavity is partially filled with fine-grained geopetaf sediment some of which is c eme nted by aragonite and some of which is cemented by Mg-calcite. The void at the top of the cavity contains a first genera· tion of botryoidal aragonite (the holes in which are due to thin section prepara· tion) and a second generation of fibrous Mg-calcite cement (scale bar 0.5 em).
Figure 32-A slab of coral rudstone from the Belize barrier reef, c e me n ted together to form a pavement of co r al conglomerate in the lee of the reef crest. T h e corals in this limestone are less than 500 years old (scale in em).
variety of growth forms also leads to the formation of many bindstones and bafflestones. Below 30 m or so, wave surge is almost non-existent and light is very attenuated. The response of many reef-building metazoans is to increase their surface area by having only a small basal attachment and a large, but delicate, plate-like shape. Rock types from this zone look like bind stones, but binding plays no role in
the formation of these rocks and perhaps another term is needed. The deepest zone of growth of cor al and green calcareous algae on modern coral reefs is around 70 m . The lower limit may depend o n many factors, perhaps one of the most im portant being sedimentation, so that this lower limit should be used with caution in the interpretation of fossil reefs. Sediments on the reef front are of
two types: ( 1 ) internal sediments within the reef structure , generally lime mud, giving the rocks a lime mudstone to wackestone matrix; and (2) coarse sands and gravels in chan nels running seaward between the reefs. These latter deposits have rare ly been recognized in ancient reefs . As a result of numerous observa tions on modern reefs, it appears that most of the sediment generated on the upper part of the reef front and
362
Reef
Figure 33-A sam ple of reef l imestone (circa 9,000 years o ld ) from the Belize barrier reef wall. The coral is a Mon tastraea cavernosa co lo ny while the sediment below is a platey algae
(Halimeda�rich
lime wackeston e to
packstone. The numerous cavities are mostly shelter voids (scale in em).
Figure 34-A polished slab of reef limestone (circa 1 3,000 years old) from the Bel i ze barrier reef wall. This algal· plate floatstone to rudstone occ u rs be tween large coral colonies and is composed of numerous Halimeda plates (white) often roofing shelter pores and skeletal packstone to wackestone.
on the reef crest is transported episodically by storms up and over the top and accumulates in the lee of the reef crest . Sediments on the in termediate and lower regions of the reef front, however, are transported down to the fore-reef zone. Shallow-
water material is contributed to the fore-reef zone only when it is chan nelled by way of passes through the reef. Reef Flat Zone The reef flat varies from a pave-
ment of cemented, large skeletal debris with scattered rubble and cor alline algae nodules to shoals of well washed lime sand in areas of moderate wave energy. The sand is mainly plates of the calcareous alga Halimeda, which grows on the reef
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363
Fi g u re 35-A small coral colony wh ict1
has been sawed in half revealing several bivalve borings, the most p rominent of which, running horizontally across the speci men, stil l contai ns the living mollusk Gastrochaena sp. This sample is fro m a patch reef on Eniwetok Atoll (photo by J. Warme).
Figure 36-Looklng down on the ce nter of a small patch reef in 2 m of water near Goulding Cay. Bahamas. At the center is a hemispherical coral (Diploria) the base of whictl has been i n tensively bored and was easily broken off with a light push of the hand.
364
Reef
Figure 37-The shore of a small island on the Belize barrier reef complex that is composed entirely of coral rubble, erod ed from the reef just offshore and swept together by currents and waves.
Figure 38-A large colony (ci rca 1 .5 m across) of Diploria that has been i njured and died in several places. Each of these areas has been infested by boring sponges (the bright orange tissue). The
shadows are the entrances to small cavities roofed over by the coral. Thi s colony is on the Flower Garden Bank i n 20 m o f water off t h e Gu lf o f Mexico coast (photo E. A Shinn).
Reef
·
365
Fi g ure 39-A polished slab of reef wall limestone (ci rca 9,000 years old) from the Belize barrier reef. This was o rig in a l ly a coral (Montastraea annularis) w h i ch has been subj e c t to seve ral g ene ra ti o n s of sediment infil l , lithification and boring by sponges. The o rig i na l coral skeleton can just be seen at the center. In this sample there a re seve ral different types of fine-grained sediment infill (light b row n and dark b row n ) as well as dif fe rent types of spong e bor i n g s (t iny holes at the l owe r right, mediu m-size galle ries at the top, and large i r reg u lar voids in the center). The process of sponge boring sediment infill and early l ithification repeated many times has almost completely alt er ed this coral to a skeletal wackestone.
Fig u re 40-A thin se ction i n pl ane pola ri zed li g ht of a coral partially altered in a manner similar to that ill ustrated i n Figure 39. Th e section is stained with clayton yellow so that a l l aragonite com ponents are white and aU Mg-calcite components are red or p i nk . The coral pores are pa rtia lly to c o mpl e t ely filled with either a rag o n it e ce me n t , M g ·ca l c i te cement, sediment, or a l l t hr ee . The ir reg u la r cavity at the center was ex cavated by a sponge (Siphonodictyon) and parti a l l y filled with fine-grained pe l o i dal lime wackestone, mostly c em e nt ed by Mg-calcite (scale bar 1 em).
Fig u re 4 1 -An idea l ized sketch of a ci r cular algal cup reef from Bermuda. The el evated rim is exposed at low tide and the depth of the surrounding coral floor is 3 to 8 m. I ndividual cup r eefs vary from 1 0 to 30 m in the i r long d i mension and the cen t ra l de pre ssio n is 2 to 3 m deep at high water (modified from Ginsburg, Sch roeder and Shinn, 1 97 1 , by permission of the Johns Hopkins Press).
366
Reef
Figu re 42-An areal view of the south coast of Bermuda with numerous algal cup offshore reefs in the lower foreground. Each cup reef has a raised rim and slightly deeper interio r The plat· form extending from shore is also rimm ed by a raised lip of similar compositio n (photo R. N. Ginsburg). .
Fi gu re 43-The margin of an isolated cup reef near the northern margin of th e . Bermuda Platform exposed at spri ng low ti d e . This cup reef rises from water 8 to 1 0 m deep and the rim is composed of an intergrowth of cora l l ine algae and vermetid gastropods (photo R. N . Ginsburg).
seaward. Sand shoals may also be range from clean skeletal lime grainstones to rudstones . present in the lee of the reef pave ment. Vagaries of wave refraction Back Reef Zone may sweep the sands into cays and islands . These obstructions in turn In the lee of the reef flat, condi tions are relatively tranquil and much create small protected environments very near the reef crest . Water over of the mud formed on the reef front comes out of suspension. This, coupl this zone is shallow (only a few ed with the prolific growth of mud meters deep at most) and scattered clumps of reef-building metazoans are and sand-producing bottom fauna such as calcareous green algae, common. The resulting rock types
brachiopods, and ostracodes, among others, commonly results in mud-rich lithol ogies. The two most common growth habits of reef-builders in these environments are stubby, dendroid forms, often bushy (Fig . 24) and knobby, and/or large globular forms that extend above the substrate to withstand both frequent agitation and quiet muddy periods. The rock types characteristic of this
Reef
367
F ig ure 44-An underwater vi ew of the margin of a cup reef looking up toward
the breaking surf. The reef Is generally
·
devo id of coral g rowth and has the a p pearance of bare rock (photo A. N. Ginsburg).
Fig u re 45-A out and polished slab of cup-reef l imestone composed of an in· tergrowth of coralline al g ae and MiJ!epora enc rusted by the foraminife r Homotrema (red). T.l'le pores and cavities are filled with cemented skeletal sand (brown) which is, in turn, coated with a rind of mangan ese (black l ines). Remain· ing cavities are filled with yet a f i n a l stage of fine-grained wackestone which i s also cemented (scale in em) (photo R. N. Ginsbu rg).
368
Reef
Figu re 46-A cut and pol ished slab of limestone from an algal cup reef com· posed of coralline algae (white layers) in which the voids between layers (black areas) are not filled with cemented sedi ment (scale in em) (photo R. N . Ginsburg).
Figure 47-An a real view of Rodriguez Key, Flo r ida show i ng a shallow bank composed of muddy skeletal sediments, fringed by a rim of branch ing corals and calcareous algae and topped by a forest of mangrove t rees. The bank is 2.5 km long and located on the inner part of the Florida reef tract i n shallow wate r : just seaward of the Florida Keys (top), a ridge of Pleistocene l i mestone (photo E. A. Shinn). ,
environment are bafflestones or floatstones to occasional framestones with a skeletal wackestone to packstone matrix.
Early Diagenesis
Two processes that tend to affect reef sediments along the platform margin in particular and that can be recognized in the rock record are ear ly cementation and bioerosion. Early
cement , in the form of Mg-calcite (as isopachous rind s of spar and as micrite) (Figs. 28, 29) and/or aragonite (as needles and large spherulites) (Figs. 30, 3 1) , has been found to be particularly common in
Reef
369
Fig ure 48-A diagram ill ustrating the su rf ac e distribution of p ri n c ipa l carbonate-produc ing skeletal organisms on Rodriguez Key Bank, Florida. The narrow ' ' reefy" rim or windward bank margin is composed of numerous small branch ing corals (Porites) and branching cora lline algae (Goniofithon). The line of secti o n t h roug h -A·A ' is illu strated in F ig u re 50 (after Tu r m e l and Swa nson, 1 976, with permission of Soc. Econ_ Paleontologists and Mineralogists).
Figure 49-A plastic im pregnated and slabbed core through t he branching cor a l (Porites porites) and calcareous algae (Halimeda and Goniolithon} seaward r i m of Rodri g u ez Key Ba n k. T o th e right of t h e core a r e fragments of t h e coral Porites porites, the wh i t e portions being
dead and tt1e upper yellow portions alive; scale i n inches (right) and cen timeters (left) (p h oto R. N. Ginsburg}.
ocean-facing reefs . These cements oc cur both in the reef itself, in the wall below the reef, sometimes in the fore reef zone, and sometimes in the con glomerates of the reef flat (Figs. 32, 33, 34). Bioerosion (Figs. 35, 36), while just as common in the lagoon reefs, can, with the occurrence of early lithifica-
tion to create ever-new substrates for infestation, with time often alter the depositional fabric of the reef limestone (Figs. 39, 40) . This process involves repeated boring by sponges, bivalves, and other endoliths , death of the organisms, infill of the cavity by sediment, lithification of that sedi ment , and then reboring by another
generation of endoliths. Algal Cup Reefs
Cup-shaped reefs (Figs. 41 to 46, 50) up to 1 0 m high and a few tens of meters in diameter are known from Bermuda, Yucatan and Brazil and oc cur on the seaward margins of pro nounced breaks in slope off islands,
370
Reef
Figure 50•-A diag ram matic c ross sect ion through Rodriguez Key Bank, . Flonda (after Turmel and Swanson, 1 976, with permission of Soc. Econ. Paleontologists and Mineralogists).
Figure 5 1 -Close vi ew of plastic i m pregnated and slabbed core through the Porites-Goniolithon zone just i n the lee of the seaward rim of Rodriguez Key Bank. The corals are Porites porites, the white rod-like skeletons are branching coralline alga Goniolithon and between arE bivalve shells, small gastropods and platey alga Halimeda. This sediment would be classified coral-coralline alga ru dstone with skeletal packstone/g rai n stone matrix.
Figure 52-A close view of a plastic slabbed core through the seaward rim of Rodriguez Key Bank. The branchi ng corals are Porites porites and al most al l of the platey algal skeletons are from the green alga Halimeda. This sedime n t would be cl assif ied as a coral-platey algal floatstone to rudstone with a skeletal packstone m atrix. i m preg n ated and
Reef
371
Figure 53-An areal view looking across Florida Bay. The sinuous features are linear mud banks topped occas i on al ly by islands which are colonized by mangrove trees. The water in the areas between banks is 3 to 4 m deep and the long spit in the foreground is about 2 km in length. The white areas off the banks are regions in which fine-grained bottom sediment has been stirred up.
Figure 54-A diagrammatic cross section through Cross Bank, Florida Bay (modified f rom Enos and Perkins, 1 979, with permission of Geol. Soc. America}.
shelves, and platform margins . The best described examples occur in Bermuda (Ginsburg and Schroeder , 1973), and in plan view these cup reefs are circular, elipsoidal , oblate or crescentic. All the examples have an elevated rim that is awash at low tide and surrounds a central depression, a micro-lagoon, usually only a few meters deep but occasionally up to 5 m deep (lams, 1 970). In three dimen sions the reefs are cup- or vase shaped, with rough inward sloping
sides. Near the bases of many cup reefs there is a sand-floored moat-like depression and numerous cavities 1 m or more in diameter extending from the moat into the base of the reefs . A first impression on inspection of the underwater surfaces of the cup reefs is the sparseness of growth on them. There are sheet-like growths of yellow-brown Millepora (a calcareous hydrozoan) often with protruding knobs a few centimeters in diameter. There are occasional smaller en-
Figure 55-A plastic-impregnated and slabbed core of sediment from one of the mud ban ks (the horizontal cracks are an artifact of s amp l e preparation}. The sediment is mostly g ray lime mudstone to wackestone with occasional small concentrations of shells or individual gastropods. The linear brown tubes are rhyzomes of the marine g rass Thalassia which commonly covers large portions of the bank tops (scale in em).
372
Reef
Figure 56-An idealized sketch of a deep-water c arbon ate mound or lithoherm from a depth of 600 to 700 m in the northeast St ra its of Florida. The sketch is constructed from observations accumulated during dives on the submersible ALVI N . An exposed hard ground (foreground) Is partially covered with a veneer of muddy carbonate sand. Attached organisms cluster on the rock mound composed of a sequence of con centric crusts of submarine lith ified sedi· ment (after N eumann et al, 1 977, with perm i ssi on of G e ol Soc. America). .
Fig u re 57-A sketch i l l ustrating the dif·
terence in geometry between a
biostrome, which is a bedded unit com pos e d of a concentration of g ene ra l ly In place ca rbonate skeletons and a bioherm which is a lens li k e body of organic orig in with i n rocks of d ifferent lithology. -
crusting and dome-shaped corals, (Diploria, Porites astreoides) , and there are shrublike growths of the brown algae Stypopodium and Sargassum. But the prevailing impres sion is that the reef is a massive limestone with a sparse cover of organisms. Closer inspection of the surface shows that it is an almost continuous living skin of crustose cor alline algae and Mi/lepora in which numerous Vermetid gastropods are imbedded . Examination of numerous specimens from the surfaces of cup reefs shows that the principle car bonate secreting constituents are crustose coralline algae, Millepora and attached Vermetid gastropods
(Dendropoma irregu/are). There are large variations in the relative propor tions of these encrusting organisms, even within large hand specimens, but lamellar growths of crustose algae and/ or Millepora with imbedded gastropods generally form well over two-thirds of the surface. By sectioning several of these algal cup reefs, Ginsburg and Schroeder ( 1 973) have found that they are built by an intergrowth of these encrusting organisms, principally crustose cor alline algae, the encrusting hydrozoan Millepora, and the attached gastropod Dentropoma irregularae. The growth framework of these algal cup reefs has extensive voids -
large and intermediate size growth framework and shelter pores, borings of bivalves and sponges, and both intra- and interparticle pores. A varie ty of vagile and sessile organisms (coelobites) inhabit these pores, with an encrusting foraminifer, Homotrema rubrum, the most abun dant attached coelobite, and the tests of a variety of benthic foraminifera and ostracods are common; branched coralline algae, barnacles, bivalves , ahermatypic corals, bryozoans and burrowing crustaceans occur in vary ing abundance. Beginning millimeters below the liv ing surface, internal sediments ac cumulate in these extensive voids.
Reef
Figure 58-A sketch illustrating the dif ference between a stratig raphic reef, which i s a 1hick, laterally restricted mass of carbonate rock, often composed of seve ral supe rimposed bioherms which individually had little relief above the sur rounding sea floor and an ecologic reef which is a rigid, wave-resistant topographic structure generally formed during one specific period of time.
Coarse-grained skeletal sand derived from the surface of the reefs is char acteristic o f the larger voids; lime mud with the tests of planktic foramini fera and planktic algae generally occurs in the smaller voids . Most specimens from the interior of the reefs show multiple generations of internal sediment that vary in grain
size, composition and color. The sand size sediments are pumped into the voids by the frequent and intense wave action; the lime mud settles out in the smaller, less agitated pores. Cementation of internal sediments and surrounding growth frame begins centimeters below the living surface. It is so pervasive that marble-hard reef rock is developed within 1 /2 m or less of the surface . The cement is principally magnesium-calcite. The rock types in these cup reefs are primarily coralline alga-coral bindstones with numerous cavities partly to completely filled with multi generation, geopetal sediment.
373
Inshore Banks of Branching Coral and Algae
These reefs are typified by a series of banks in the nearshore zone along the Florida Keys (Figs. 47 to 52) which are covered by a community of calcareous algae and branching cor als. The banks, up to 3 km long and 1 km wide, and oriented more or less parallel to the F1orida Reef tract, rise as much as 4 m above the surroun ding sea floor and are exposed during spring low tide. They are protected from ocean swells by numerous reefs and shoals, but they are washed by wind waves from the northeast . All of the banks show a marked zonation (Fig. 47) . The windward margin (east facing) is a zone of
374
Reef
Wave Energy
GROWTH FORM
�....
Delicate, branching
low
high
-.ms
;o;�IC �
Thin, delicate, plate-like
low
low
.•. ,
Globular, bulbous, columnar
moderate
high
mod-high
moderate
mod-high
low
Intense
low
moderate
low
b It & Fi g u re 59-A sketch illustrating the growth form of reef-building metazoans
and the types of environments in which they most commonly occur.
-�
.......
Robust, dendroid, branching Hemispherical, domal I rregular, massive
Encrusting Ta bula r
STAGE
TYPE OF LIMESTONE
SPECIES DIVERSITY
SHAPE OF REEF .BU ILDERS
DOMINATION
blndstone to lramestone
low to moderate
Laminate encrusting
high
domal massive lamellar
framestone (bindstone) DIVERSIFICATION
mudstone to wacke&tone malrix
Fig u re 60-A sketch of the four divisions of the reef core facies with a tabulation of the most common types of limestone, relative species diversity and shape of reef-builders found in each stage (after James, 1 979, with permission of Geol. Assoc. Canada).
branching finger corals (Porites porites var. divericata) and branched twig-like coralline algae (Goniolithon strictum). The top of the banks are gardens of marine grass (principally Tha/assia testudinium), calcareous green algae (Halimeda, Acetabularia), infaunal bivalves, and burrowing crustaceans. Off the bank in the sur rounding water is a similar communi ty, but with numerous echinoids and large sponges as well . Many banks have a composite in ternal structure, early Holocene mud bank cores veneered with 2 to 3 m of coral/algae deposits (Fig. 50), while others are all corals and algae.
Sedimentation
COLONIZATION
to floatstone with a mud matrix
STABI LIZATION
grainstone lo rudstone (packstone to wackestone)
Because most of the organisms that contribute to these banks are branch ed and segmented, they commonly break down on deposition, leaving no evidence of a coral and algal " frame. " Cores through these banks show that most of the windward fringe is recorded as a branching cor al floatstone to rudstone with a Halimeda (platy green algae) and Goniolithon (branching red algae) , grainstone to packstone matrix. Sub surface sediments on the bank top are burrowed Halimeda and/ or Goniolithon and bivalve packstones to wackestones.
branching
encrusting
low
lOW
skeletal de bris
Linear Mud Banks
Accumulations of lime mud (Figs. 53 to 55) that stand some 3 to 4 m above the sea floor have been documented in Florida Bay and on the northern shelf of Belize. Florida Bay is compartmentalized into a series of " lakes" by shallow, sub-linear mudbanks. The sea floor in the lakes consists of a winnowed lag of molluscan shell fragments and/or a thin veneer of bioturbated mud over Pleistocene bedrock . The tops of the banks are awash at low tide and covered by a thick growth of the sea grass Thalassia testudinium. They are occasionally
Reef
PERIODS TE RT I A R Y
B I O HERMS
MAJOR S K ELETA L E L E M E N TS 6
C O R A LS
4
CORALS
T
(/) z 0 ..J ...J -
375
sponges stroma toporoids
:!
z (/) - a:
w
!i � -
R EEFS
..... LL
uo C) 0 -
STR OMATO P O R O I D S
corals
s
b ryozoa
..J
0
skeletal a
Cl
skeletal algae
w
A R C H A EOCY ATH I DS
Figure 6 1 -An idealized stratigraphic
column representi n g the Pha n e rozoic
and ill ustrating times when there appear
:o be no reefs or biot l€lf1 1S (ya.p::;), times when there were only reef mounds, and times when there were bo;h reefs and reef mounds; tl1e numbers indicate dif· ferent associations of reef-b u i lding taxa di scussed in the text .
Fig u re 62-A cross section through a hypothetical reef-mound i l l ustrat ing t h e geometry of t h e different facies and stages, based on Wilson ( 1 975) (after James, 1 979, with permission of Geol Assoc . Canada)
REEF M O U N D Cap
ae
0
e
376
Reef
Fig u re 63-A dense region of club shaped stromatol ites in the intertidal zone of Hamelin Pool, Shark Bay, Western Aust rali a (photo P. Playford).
topped by islands and often cut by tidal channels. Banks that intercept effective winter storms have different windward and leeward sides (Fig . 54); the windward sides have steep slopes mantled by a skeletal lag, while the leeward have a gentle slope and a thick carpet of grass associated with muddy sediment. Bare areas with positive relief are traction deposits of pelleted mud with a distinct surface asymmetry, ornamented by low relief ripples. These areas are most com mon in the form of spits that project from the leeward sides of mud banks. Most of the sediments are skeletal wackestones thoroughly burrowed and penetrated by roots and rhizomes of turtle grass (Tha/assia). Deep Water Carbonate Buildups
Figure 64-A cross section through a columnar algal st romatol ite from the in tertidal zone of Hamelin Pool, Shark Bay. Western Australia (photo J . L . Wray).
At the other end of the spectrum, a combination of seismic, dredging and direct observations from submersibles indicates the presence of elongate mounds up to l 00 m long and 50 m high in depths of 600 to 700 m in the
Reef
Figure 65-A ciuster of stromatolites in 3 m of wat e r on the floor of Hamelin Poo l , Shark Bay, Western Australia. The "beard" a round the lower parts of the columns is mostly the calcareous alga Acetabularia and the rippled sand is mai n lv oolitic lohoto P. Plavfordl
straits of Florida (Neumann, Kofoed, and Keller, 1 977) (Fig . 56). These mounds are composed of surface hardened concentric crusts of sub marine lithified muddy to sandy sedi ment and have therefore been called "lithoherms . " The mounds rise from a flat sea floor composed of hardgrounds veneered with rippled, muddy skeletal sand (pelagic forams and pteropods) . The most common biota are stalked crinoids and sea pens. The crinoids are oriented into the bottom currents that flow at rates of from 2 to 7 em/sec. The mounds themselves have steep sides, a pronounced northward elongation , smooth margins, and ir-
s
ATHAPUSCOW AULACOGEN
377
SLAVE CRATON
0 greywoc"- tvtbidtttu
11&1 conOf)hytic
Moristone
S LOPE FACIES
.B. lil'ne1tor.e rhythmite1![ wlrh brecc.io beds
N
.. biohennal 1tromotolite-s
m rolumnar stromotoliJtt.J m non-co5'1JMttOJ SfOtnOtOJites - <>Diiti' lime-stone
Figure 66-A diagrammatic c ross sec tion illustrating facies transitions in the
regular tops. The crest is commonly populated by a dense community of deep-water organisms such as unstalk ed crinoids, branching ahermatypic corals, sponges and alcyonarians.
Pethei Group of early Proterozoic age fro m the Sl ave craton into the Athapuscow aulacogen (mod i fied from Hoffman , 1 974).
378
Reef
Figure 67-An outc rop photograph of pan or Gl 20 rn l� tick t.Jiostrorne of l ow relief (la!e(ally linked) columnar stromatolites i n a back-reef, littoral en viron men t . The section here is perpen dicular to the �1ongat1on ot the columns i n plan. Wildb read Formation. ea rly Pro �erozolc . East A•m Great Slave Lake, North West Territories, Canada (photo P.
Hoffman).
Individual layers or crusts are 10 to 30 em thick and conform to the shape of the mound. The top of each crust is smooth. The sediments are more cemented at the exposed upper sur face and become progressively less cemented and more irregular toward the base. Voids between crusts pro duced by winnowing out of soft sedi ment and by boring , if later filled with spar, may resemble some
stromatactoid-like structures in the rock record. The rock ranges from a mud supported to grain-supported coral biomicrudite to pelagic foraminifer pteropod biomicrite to biopelmicrite. Micropelleting is common in the sediments and the rock is character ized by manganese stains, geopetal fills, and multi-generation boring and sediment infill . The sediments are cemented by Mg-calcite micrite and intensively bored by sponges . FOSSIL REEFS Nomenclature
While it is easy to recognize a modern reef or bank while swimming or flying over it, in the rock record it
is an entirely different matter. There are several reasons for this, but the main complicating factor is time. First, most of our observations are in the vertical dimension, in quarry walls, in mountain exposures, in roadcuts, in drill core, or in cross sec tions, all of which show the reef as a limestone body constructed of dif ferent components generally formed at different times. Second, organisms have changed and evolved with time, so that the reef-building metazoans for one period are different from those in another period. Another complicating factor is diagenesis, the main component of the reef that allows us to differentiate stages of reef growth and different facies may be dolomitized at best or completely
Reef
dissolved away at worst. The final compromise is exposure - cover, tec tonics and partial well control often result in only a partial picture of the whole structure, and so the true geometry of the body cannot be determined with certainty. The upshot of all these is that the description of a reef or a reef-like structure in the rock record should be approached with caution. Perhaps the best and most rigorous approach is to first describe the shape and size of the limestone body and then, if possible, on the basis of detailed examination of the internal structure, postulate as to its origin , evolution and structure. Another factor that has led to con fusion in nomenclalture results from the misconception of scale. A com plete reef in the Lower Paleozoic or Cretaceous which is formed by a variety of organisms and displays
several stages of growth may be only as large as a single coral colony in a Jurassic or Pleistocene reef. Because of these complications, the problem of fossil reef nomenclature is one that has pervaded the literature on carbonate rocks for over 80 years . For detailed reviews of this problem, the interested reader is referred to scholarly treatments by Nelson and others ( 1 962) , and Heckel ( 1 974). The word bioherm has long been a useful non-genetic term for lens-like bodies of organic origin that are embedded in rocks of different lithologies (Fig . 57) (Cummings, 1 932). These structures should be dif ferentiated from biostromes, which are purely bedded structures, such as shell beds, coral-rich beds, and the like, consisting of and built by seden tary organisms that do not swell into mound-like or lens-like forms (Fig .
379
Figure 68-An outcrop photograph of a biostrome composed of isolated high relief columnar stromatoids. The i nter column areas are filled with edge-wise i ntraclasts. Wildbrcad Formation, Early Proterozo ic. East Arm Great Sl ave Lake , North West Territories, Canada (photo P . Hoffman).
57}, Wilson ( 1 975} uses the term "carbonate build-up" for a body of locally formed (laterally restricted) carbonate sediment which can be demonstrated to possess topographic relief. This term likewise carries no inference as to the internal composi tion or size of the structure; for ex ample, a carbonate build-up may be composed of both bioherms and biostromes. Dunham (1 970) suggested separating the different types of car bonate build-ups in the rock record into two types. The first of these he called stratigraphic reefs or thick
380
Reef
Figure 69-Th e flank of a small (3 m wide) bioherm of columnar stromatoids frorn a tidal i:... ha nnei between rnuch la rger :Oioherms. Sediment adjaoe�t to t e bioherms is c ross-bedded in traclast grai nstone (scale in tenths of feet). Ta lthei lei Format ion , Early Pro[erozoic, East Arn, G r·eat Slave Lake, Nor t11 West Tcrritoncs, Canada (photo P. Hoffman). . •
laterally restricted masses of pure or largely pure carbonate rock (Fig . 58). Implicit in this concept is its objec tivity and reference only to the geometry of the build-up . On the other hand, an ecologic reef is a rigid, wave-resistant topographic structure produced by actively building and sediment binding organisms (Fig. 58). The binding in this structure must be organic and not inorganic . A stratigraphic reef may
include ecologic stages . When viewed in the experience of the rock record, there is in fact a large class of structures which could be called stratigraphic reefs and com monly comprise a succession of localized biostromes and/ or bioherms which had little or no relief above the sea floor at their time of growth (Fig . 58). While the term ecologic reef is often used, the prerequisite that it be an organically-bound, wave-resistant topographic structure is unsettling. Thi s concept comes from a discussion by Lowenstam ( 1 950) in his classic paper on the Silurian reefs of the Niagara region , where he felt that the biologic potential of frame building and sediment binding organisms to
actually build wave-resistant struc tures defined a reef. If this concept were rigidly applied, many modern reefs would not qualify because most are not organically bound, especially those in the platform interior and deep water (where there are no waves) . In addition , it is quite clear that the bulk of many reefs is a pile of in-place coral rubble and sediment which are certainly not wave resis tant . On the other hand, fossil buildups composed of delicate bran ching organisms which would appear on the surface to have no wave resistant potential may have been cemented very early and so h ave been hard structures of limestone on the sea floor at their time of growth. With these facts in mind, the term
Reef
381
7 0-An outc rop photograph of the late r a l ma rgin ol a large (2 m wide by 1 5 m hig h , but l ess than 2 m synoptic relief) bioherm of col u m nar stromatoids. The b i oherms a re e;ongate, perpen dicular to depositional str ike as a re the const!tuent columns . Ta ltheilei Formu· tion, Ea rl�, Proterozo i c , East Arm Grea t
Figure
ecologic reef should be used with cau tion and rigorously defined . Perhaps a useful alternative is use of the term stratigraphic reef as defined , but use of the terms reef and reef mound only as discussed in the next section , once the detailed com position of the structure is known. Analysis of Reef Limestones
At this point , it should be clear that to interpret any fossil reef or reef complex, the key is to extract as much information as possible from the limestones that make up the reef core . A concise description of the type of reef limestone is a necessary first step. Several other observations, however, such as ( 1 ) the relative diversity of reef-building organisms, (2) the growth form of the reef builders, and (3) the nature of the in-
ternal sediments and cements are also needed to give a complete picture. Diversity Among Reef-Building Organisms Very diverse faunas in terms of both growth form and taxa occur when a community is well-established and conditions for growth are op timum, that is, nutrients are in good supply, while chemical and physical stresses are low. In such optimum en vironments , the division of biomass among various species is due mainly to complex biological controls. In contrast, low diversity en vironments commonly fall into three general categories - unpredictable environments; new environments (fauna moving into a new environ ment), and severe environments (high chemical and physical stress) . Among
Slave Lake, North West Territories, Canada fohoto P. Hoffman).
the factors most likely to stress modern and fossil reef-building com munities are : ( 1 ) temperature and salinity fluctuations - most modern and likely most ancient reef-builders grow or grew best in tropical sea water of normal salinity; (2) intense waves and swell - the skeletons of most reef-builders will be broken or toppled by strong wave surge; (3) low-light penetration - in modern reef-building organisms, rapid calcification takes place because sym bionts, which are light dependent,
382
Reef
Figure 7 1 A bedding su rface of elongate r.o l u m n a r s:-romfi1olites f!Om
20 rn th ick undulcse biostrome which grades seaward a few k i lometers i n to
a
discrete or l i f1ked bioherm s . T al ! hei lei
Proterozoic, East Arm Great Sl ave Lake , N o rth Wesi Ter ritories, Canada (photo P . Hoffman).
Fo rma tion , Eariy
take over some of the bodily func tions of the host; and (4) heavy sedimentation - all reef-builders are sedentary filter-feeders or micro predators and water filled with fine grained sediments would clog the feeding apparatus.
The Growth Form of Reef-Building Metazoans The relationship between organism shape and environment is one of the oldest and most controversial topics
in biology and paleobiology. In terms of reef-building metazoans, however, many observations of the interrela tionship between organisms and sur rounding sediments from the rock record combined with studies of modern coral distribution on tropical reefs allow us to make some generalizations about form and en vironment that are very useful in reef facies analysis (Fig. 59) . Internal Sediment and Cement The identification of internal sedi ment , or that sediment that has filtered into, and partially to com pletely filled cavities in the reef, is often a key to identification of reefal carbonates . The most common ex pression of this sediment is its
laminated geopetal nature with the overlying void now occluded with ce ment. This may range from an ob vious filling between corals or stromatoporoids to the much more subtle partial filling of a cavity in a mudstone mound with a mud of a slightly different composition. Documentation of internal sediment of clearly marine origin is good evidence that there were holes or cavities in the structure. This in turn implies that the structure probably stood above the sea floor and that there was a structure formed by an ir regular accumulation of large skeletal metazoa and/or some combination of smaller metazoans, early lithification, and possibly dewatering . Early lithification is common in
Reef
modem reefs and also appears lO been widespread in anciem reefs as well . This '-:ement is often in the form of tiny micrite-size crystallites and is therefore almost impussible Lo dffferentiate from mud in the rock record. More easily recognizable is radiaxial (Bathurst, 1 959), fibrous or botryoidal calcite associated with the voids in reef rock. Because these cements (or their Mg-calcite and aragonite precursors) may also be precipitated in environments other than on the sea floor, great care should be exercised before identifying them as syn-sedimentary. Among the criteria that can be useful in this regard are: { 1 ) Is this cement localized to the reef and not present in the sur rounding strata?; (2) Is this cement have
383
Figure 72-An outc rop photog ri:iph of a bioherrnai ci uster of deep-water stromatoids in fore-slope rhythmites composed of brown terrigenous me�r lslone, yellow allodapi c dolomite and gray carbon ate. Pen 1 0 em in length at top right for sca le. Mclean Formation. Early Proterozoic . East Arm Great Slave Lake. North West Territories. Canada (photo P. HoHman).
interlaycred with internal marine sedi i n cavities'! (3) Are organisms seen growing on t.h.e cement as a su bstrate? ; (4) I s the cement cut b y btJi i ng� ; (5) Arc da�t s with this cement or clasts of this cement seen in surrounding off-reef sediments? ; and (6) If this cement is finely crystalline, are the cemented sediments multi generation and cut by numerous bor cement now neomorphosed to radiax . mgs . ?. ial or fibrous calcite (Bathurst, 1 959) . Among common structures in many Internal Structure of Fossil Reefs Paleozoic reefs is stromatactis. Once thought to be a recrystallized reef It has long been recognized that there is an ecological succession in building organism, these structures are now generally regarded as narrow many Paleozoic reefs (Lowenstam, sedimentary cavities with flat bases 1 950), that is, the replacement of one and irregular roofs that are community of reef-building organisms sometimes floored with internal sedi by another as the reef grew. A recent ment but filled mostly with submarine synthesis by Walker and Alberstadt ment
384
Reef
Figu re 7 3 -An a real view, looking down on a Lower Cambr ia n bioh e r m exposed on the low tide platform at Point Amour. Labrador , Canada. The mo u n d s h ap ed bioherm (center), composed mostly of arctlaeocyathans, is a bou t 8 m in diameter. The surrounding joints are i n -
i n t�r.-se�fJi.n:.�.1?!Qt:J_��- en_d .§h_a l �s_, --
Fig u re 74-A Lower Cambrian bioherm exposed in cliffs along the Strait of Belle Isle, Labrador, Canada_ The white nodu lar l imestone of the bioherm is rich in a rchaeocyathans .
Reef
385
Figure 75--The margin of a Lower Cam brian bioherm complex exposed in cliff section along the shores of the Strait of Belle I sle, Labrador. The i rreg u lar nodular l imestone to the right i s the reef complex and the massive well-bedded sediments to the left are off-reef rudstones composed of skeletal debris. These beds g rade rapidly into inter-reef shales and nodular limestones at the far left. Figu re 76-The surface of an ar chaeocyat han biostrome i n southern Labrador . This plan view i l lustrates that almost all of the cup-shaped ar chaeocyathans a re in upright posit ion . The sed i ment between skeletons in this archaeocyathan bafflestone is a skeletal lime wackestone.
386
Reef
Figure 77-A close view of sheet-like, prone a rchaeocyathans at the bas a l . pioneer stage of an archaeocyathan biost rome in western Newfo u n dland,
Canad a .
Figure
78-A slabbed a n d polished por· tion of t he a rchaeocyat han
floatstone/bafflestone that makes up the Lower Cambrian bioherms from southern
Lab rador_ The white skeletons a re ar chaeocyathans, the white speckles Renalcis and the dark red sedi m ent skeletal wackestone to packstone. At t h e lower right is a cavity filled with geopetal sediment (scale in em).
Reef
387
Figure 79-A close view of a partially filled cavity from one of the Lower Cam brian bioherms in southern Labrado r . Clusters o f Renalcis hang from t h e roof and a re in turn encased in fibrous calcite cement that has developed a botryoidal appearance. The geopetal sediment at the base is interlayered with cement and has buried some of the botryoids that grew down from the roof (rig_ht center) (scale in em).
Figure 80-An outcrop photograph of the top of a small mound in one of the Lower Cambrian bioherms from southern Labrador. The dark spots a re the tops of tubular borings (Trypanites) that have been fil led with terrigenous silt and dolomite (scale in em); note that the bor ings penetrate both skeletons and matrix, indicating that the sediment was l ithified.
( 1 975) of reefs ranging in age from Early Ordovician to Late Cretaceous suggests that a similar community
tions on modern reef communities with some measure of confidence. In most cases, four separate stages
Pioneer (Stabilization) Stage
This first stage is commonly a series of shoals or other accumula
succession is present in reefs
of reef growth can be recognized , and
tions of skeletal lime sand composed
throughout the Paleozoic and
these stages, along with the types of
of pelmatozoan or echinoderm debris
in the Paleozoic and Mesozoic , and
Mesozoic. Application of this concept
limestone, relative diversity of
to Oligocene reefs (Frost, 1 977) which
organisms and growth form of reef
plates of calcareous green algae in the
are dominated by scleractinian corals
builders in each , are summarized in Figure 60.
Cenozoic. The surfaces of these sedi
(the reef-builders in today's oceans)
ment piles are colonized by algae
allows us now to equate ancient reef
(calcareous green) , plants (sea
community succession with observa-
grasses), and/or animals (pelmato-
388
Reef
Figure 8 1 -The su rface of one of the
flanking skeletal calcarenite beds that s u r rounds a small Lower Cambrian bioherm in southern Labrador. The rock is a pe lmatozoan, t r i lobite g r a instone to rudstone. The pelmatozoan grains a re the white fragments in the sedimen t (scale in em).
Fig u re 82-An algal bioherm , composed of st romatolitic algae in the Middle Cam
brian March Point Formation, western Newfoundland, Canada.
Figure 83-The upper su rface of a sma l l algal bioherm i n the Whee ler Shale, M id dle Cambria n , weste rn Utah . The struc t u re is composed of stromatolitic algae and Renalcis.
Reef
389
Figure 84-A single algal head from a series of algal bioherms (background) of Upper Cambrian a g e , west New· foundland. This head is a th rombolite (Ai t ke n , 1 967) because il i s not
lami nated and has a clotted and burrow ed internal structu re.
Figure 85-A th i n section photomicrograph u n de r plane polarized
light of Epiphyton, geopetal sediment, and calcite cement from an Upper Cam brian bindstone, western Newfound land . The branching Epiphyton g rows in tiny clusters and is often associated with the laminar form Girvanella (scale 1
em).
F i gu re 86-A bioherm (called Lechuguilla Mound) exposed in the lower part of the M c Kel l ig an Canyon Forma tion (Lower Ordovician) southern Franklin Mountains at E! Paso, west Texas. Th is structure is approximately 1 5 m wide and composed of l ith i stid sponges (Archaeoscyphia), receptacu litid a lgae (Calathium), and th e enigmatic en· crusting coelenterate Pulchrilamina (photo D. Toomey).
390
Reef
Figu re 87-An outc rop photograph of Pulchrilamina Spinosa, one of the prin· c ipal biotic components of Lower Ordovi· cian mounds in the southern U nited States (photo D . Toomey).
Figure 88-An outcrop photograph of numerous sil icified specimens o f the receptac u litid alga Calathium from Lower Ordovician reef mounds i n west Texas; coin is 2 .4 em in diameter (photo D. Toomey).
zoans) that send down roots or hold fasts to bind and stabilize the substrate. Once stabilized, scattered branching algae, bryozoans, corals, soft sponges and other metazoans begin to grow between the stabilizers. Colonization Stage This unit is relatively thin when compared to the reef structure as a whole , and reflects the initial col onization by reef-building metazoans. The rock is generally characterized by few species, sometimes massive or
lamellar forms but more frequently thickets of branching forms, often monospecific. In Cenozoic reefs , the one characteristic common to all cor als in this stage of reef growth is that they are able to get rid of sediment and clean their polyps, and so are able to grow in areas of high sedimentation . The branching growth form creates many smaller suben vironments or niches in which numerous other attached and en crusting organisms can live - form ing the first stage of the reef
ecosystem. Stromatactis is common in rocks representing this stage. Diversification Stage This stage usually provides the bulk of the reef mass and is the point at which most pronounced upward building towards sea level occurs and easily definable, lateral facies develop. The number of major reef building taxa is usually more than doubled, and the greatest variety in growth habit is encountered. With this increase in form and diversity of
Reef
391
Figure 89-An outcrop photograph of the lithistid sponge Archaeoscyphia (above quarter) and Pulchrilamina spinosa (right) from one of the Lower Or dovician reef mounds in west Texas; coin is 2.4 em in diameter (photo D . Toomey).
framework and binding taxa, comes increased nestling space (surfaces , cavities, nooks and crannies) leading to an increase in diversity of debris producing organisms.
Domination Stage
The change to this stage of reef growth is often abrupt . The most common lithology is a limestone dominated by only a few taxa with only one growth habit, generally en-
crusting to laminated. Most reefs show the effects of surf at this stage , in the form of beds of rudstone. The reason for this ecologic succes sion is at present a topic of much debate. Some workers feel that the
392
Reef
Fig u re 9 1 - A n outcrop p ho tograp h of part of the lower third of the Meiklejohn bioherm composed o f "Zebra Limeston e " made up of layers of lime m udstone and calc ite spar .
Fi g u re 92- T wo pol i shed slabs ot "Zebra Limestone" i llustrating dark
g ray-brown lime mudstone, cavity-fi l l ing gray-green geopetal sediment, and two stages of radiaxial calcite cement , one dark brown and the other white (photo R. J. Ross).
Figure 94- The
surface of a s m al l
bioherm like the one from the Deane Quarry (Fig. 93) exposure ill ustrating
abundant b r yozoa n skeletons (white) i n red l i m e mudstone.
Figure 93-A smal l bioher m (recessively we a t h e r ing mass i ve limestone at ce n ter} i n the Hol ston Fo rmation , M iddle O rdovi cian (Ch azy Stage), Deane Quar ry, Ten n essee . The bioh e r m is composed al mo st entirely of bryozoans while t h e s u r r o und ing massive calca renites (cut by the row of d r i l l holes) are predo m i n a ntly pel matozoan grainstones (hammer at the cen t e r i s sca l e) .
control is extrinsic and reflects a pro gressive replacement of deep-water communities by shallower water ones as the reef grows to sea level and into more turbulent water, yet there is often abundant evidence that the first two stages are developed in shallow water. Other workers feel that the control is intrinsic and reflects a
natural succession as the organisms gradually alter the substratum and change the energy flow pathways as the commumity develops, yet there is abundant evidence of increasing water turbulence as the structure grows. Superimposed Reefs Reef structures in the rock record are often impressive because of their size, not only laterally but vertically . Careful examination of stratigraphically thick reefs, however, often reveals that they are not a
Reef
393
Fig u r e 95-An outcrop photog raph of the numerous superi mposed sheets of bryozoans (Batostoma) comprising a smal l bioh erm in the Laval Fo r mation (Chazy Stage) near Montrea l , Quebec.
Figu re 96-A small coral bioherm in the Lou r des Limestone, M i ddle Ordovician ( B l a c k River Stage), western Newfoundland.
single structure, but a series of superimposed or stacked reefs that grew on top of one another in more or less the same place. Individual episodes of reef growth are common ly separated by periods of exposure, reflected in the rock by intensive diagenesis, calcrete horizons, or shales (paleosols). When the ocean floods one of these surfaces that has been exposed , reef growth begins at the diversi fic ation stage because there is already a hard, often elevated , substrate present . Reef Mounds
This model of reef development is predicated on the assumption that a full spectrum of reef-building organisms are present, as we see in the tropical oceans today - but such was not the case for much of the Phanerozoic. The critical element that is often missing, and without which the four stages of development in the reef core cannot occur, is the presence o f' skeletal metazoans that secrete large robust, branching, hemispherical or tabu lar skeletons .
Without them the reef cannot exist in the zone of constant turbulence, usually wave induced, because smaller and more delicate forms would be broken and swept away (unless sub marine cementation is very rapid, per vasive and near-surface). This zone of turbulence is the optimum area for growth and diversity because sedi ment is constantly removed , water is clear , and nutrients are constantly swept past the sessile organisms. Such large skeletal metazoa were, h owever ,
present only at certain times during the Phanerozoic, and each period has its own specialized group of frame builders: ( 1 ) Middle and Upper Or dovician - bryozoa, stromatopo roids, tabulate corals; (2) Silurian and Devonian - stromatoporoids, tabu late corals; (3) Late Triassic - corals, stromatoporoids ; (4) Jurassic - corals, stromatoporoids; (5) Middle Cretaceous - rudist bivalves; and (6) Middle and L ate Tertiary - scleractinian corals (Fig . 61).
394
Reef
F igu r e 97-An outcrop photograph of the interior of a bioherm in the M iddle Ordovician Lourdes Limestone from western Newfoundland. The framestone Is composed of large corals (Labyrln thites) sometimes encrusted by stromatoporoids (Labechia) as at the cente r . The sediment between the corals and stromatoporoids is skeletal packstone to grainstone (scale in em).
Fig u re 98-A s m all Silurian (early Wenlock) bioherm about 2 m h igh com p osed of corals and stromatoporoids i n t h e upper Visby shales, Gotland, Sweden (ohoto P. CoOO!OJf).
What then of the rest of the Phanerozoic record - were there no reefs? While there were certainly periods when no reefs at all formed, these periods were generally short and represent either climatic/tectonic crises or the complete lack of any reef builders, even small ones (as during Middle and Upper Cambrian). During most of the Phanerozoic there were structures that some workers call reefs , some call mounds, some call banks; they lack many of the charac-
teristics we ascribe to reefs, yet they were clearly rich in skeletal organisms and had relief above the sea floor . The origin of these structures, which I have called reef mounds, has prob ably caused more discussion than any other topic in the literature on reefs (Heckel, 1 974) . When viewed against the backdrop of the general reef model , however, I think of them as half-reefs or incomplete reefs because they represent only stages one and two of the model. These structures
did not develop the other upper two stages either because the environment was not conducive to the growth of large skeletal metazoa, or because these larger metazoans simply did not exist at the time the structure formed. Reef mounds are, as the name sug gests, flat lenses to steep conical piles with slopes up to 40° consisting of poorly sorted bioclastic lime mud with minor amounts of organic boundstone. With this composition they clearly formed in quiet water en-
Reef
395
Figure 99-A view of the Flathead Range in the Canadian Rocky Mountains illustrating dolomitized bioherms (R) in the Upper Devonian Peechee member of the Southesk Formation (photo B. Pratt).
Figure 1 00-A view of t h e margin of the Upper Devonian Miette Reef complex at Marmot Cirque in the Canadian Rocky Mountains (view to the northwest). The complex comprises (1) a basal platform carbonate unit, the Flume Formation. overlain by (2) the massive light-colored dolomites of the reef margin, the Cairn Formation. that are in turn bu ried by (3) the basinal shales of the Perdrix Forma tion (photo E W. Mountjoy).
vironments and from the rock record appear to occur in three preferred locations; (1) arranged just downslope on gently-dipping platform margins; (2) in deep basins; and (3) spread widely in tranquil reef lagoons or wide shelf areas . When viewed in sec tion, reef mounds display a similar facies sequence in each case (Wilson, 1975) (Fig. 62). Reef Mound Core Facies
Stage 1 - Basal bioclastic lime
mudstone to wackestone pile; very muddy sediment with much bioclastic debris but no baffling or binding organisms. Stage 2 - Lime mudstone or baf flestone core; the thickest part of the mound, consisting of delicate to den droid forms with upright growth habits in a lime mudstone matrix. The limestone is frequently brec ciated , suggesting partial early lithification, dewatering and slump ing , and contains stromatactis. Each
geologic age has its own special fauna that forms this stage: Lower Cam brian - archaeocyathans; Middle to Lower Ordovician - sponges and algae; Middle Ordovician, Late Or dovician , Silurian, Early Car boniferous (Mississippi) - bryozoa; Late Carboniferous (Pennsylvanian) and Early Permian-platey-algae; Late Triassic - large fasciculate dendroid corals; Late Jurassic - lithistid sponges; Cretaceous - rudist bivalves (Fig. 61).
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Reef
Figure 101-An areal view of Winjana Gorge and the Napier Range reei com plex, Canning Basin, Western Australia. The soft shales that filled the basin (upper left) and covered the reef complex have been removed by erosion so that the carbonates are exhumed and are ex posed now much as they were in the Upper Devonian (photo P Playford).
Figure 102-0ne wall of Winjana Gorge illustrating from right to left: horizontally bedded back-reef and reef-flat strata; massive reef margin; and steeply dipping fore-reef sediments. Upper Devonian Napier Range reef complex, Canning Basin, Western Australia (photo P. Playford).
Stage 3 - Mound cap; a thin layer of encrusting or lamellar forms, occa sional domal or hemispherical forms , or winnowed lime sands. Reef Mound Flank Facies
These massive, commonly well bedded carbonates comprise extensive accumulations of archaeocyathan, pelmatozoan, fenestrate bryozoan, small rudist, dendroid coral, stromatoporoid , branching red algae or tabular foraminifer debris and
chunks of wholly to partly lithified lime mudstone . Volumetrically these flank beds may be greater than the core itself and almost bury it. While in many reef mounds the core is massive carbonate, in others, particularly stratigraphic reef mounds, the core is a heterogeneous assemblage of mound-shaped or pillow-shaped masses , commonly 0.5 to 1.5 m across and 0.3 to 1 .0 m thick. Smith ( 198lb) has proposed the term "saccolith" as an epithet to
describe these irregular, sack-shaped , pillow-shaped or bun-shaped, discrete, boulder-size masses of autochthonous reef rock, of whatever composition within a reef or reef complex . These elements are most easily seen in natural outcrops where weathering has accentuated mutual contacts. In summary, there are times when the model is not applicable because there are no reefs at all, there are times when only reef mounds form, and
Reef
Figure 103
397
An upper Devonian reef
marg i n at Najee Cave, Bugle Gap, Can ning Basin, Western Australia. The
massive reef front limestones (left) inter finger with the well-bedded inclined fore
reef strata (right) (photo P. Playford).
Figure 104-The
abrupt but
interfinger
ing transition in the lower 150 m of a dolomitized upper Devonian reefal car bonate platform margin (Fai rholme Group), and basinal shales and limestones (Mount Hawk Formation) at Burnt Timber in the Front Ranges of the southern Canadian Rocky Mountains (photo I. A. Mcllreath and G. E. Tebbutt).
there are times when both reef mounds and reefs occur, but in dif ferent environments. EXAMPLES OF FOSSIL REEFS Stromatolite Buildups During the Precambrian and earliest Paleozoic, prior to the ap pearance of herbivorous metazoa, stromatolites formed impressive
buildups. These stromatolite com plexes clearly had relief above the sea floor and in terms of morphology were surprisingly similar to later skeletal reefs . There are no stromatolite bioherms documented from modern oceans, but in Shark Bay, Western Australia, where all environments are hyper saline and stromatolites abound, the interrelationship between stromatolite
morphology and environment has recently been documented (Hoffman, 1 976). In the intertidal zone columnar to club-shaped forms, up to 1 m high are found rimming headlands (Figs. 63, 64) . In relatively high energy, ex posed environments the relief of the columns is proportional to the inten sity of wave action. These grade laterally away from the headlands to the lower energy bights where the
398
Reef
Figure 105-A disphyllid coral colony (Syringopora) in growth position and en cased in predominantly nodular Alveolites and lamella r stromatoporoids from the core of a bioherm in the Mercy Bay Member of the Weatherall Forma tion (upper Devonian), Banks Island, North West Territories.
Figure 106-Massive domal stromatoporoids and stromatoporoid rub ble in the upper few meters of a massive biostromal unit in the Upper Devonian Cairn Formation, Slide Creek, Jasper Na tional Park, Canadian Rocky Mountains (photo E. W. Mountjoy).
Figure 107-A close view of a coral (white) being encrusted by a stromatoporoid (dark gray) in a small bioherm in the Upper Devonian Blue Fiord Formation, Elle smere Island, North West Territories , Canada.
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399
Fig u re 108-The Upper Devonian reef flat facies at Winjana Gorge, Canning Basin, Western Australia, composed of the stromatoporoid Actinostroma, the alga? Renalcis (speckled) and laminated internal sediment (photo P. Playford).
Figure 109-An outcrop photograph of thin platy stromatoporoid framestone of the reef facies from the Silurian West Point Formation, Gaspe, Quebec; ham· mer for scale (photo P. Bourque).
stromatolites are more prolate and
mats only grow on stabilized
posed layers of mat and sediment
elongate, oriented normal to the
substrate, thus columns are nucleated
turns the structures into resistant
shoreline. In tidal pools, digitate col
upon pieces of rock; growth is
limestone. Moving sand continuously
umnar structures abound.
localized there and does not occur on
scours the bases of the stromatolites.
These growth forms are the result of active sediment movement; algal
the surrounding shifting sands. Early lithification of the numerous superim-
The mounds or pillars are largest in subtidal or lower intertidal en-
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Reef
Figure 110-A polished slab of reef lime stone from the Upper Devonian
Lloyd Hill platform atoll-pinnacle reef composed of laminar stromatoporoids, the alga? Renalcis (speckled) and a large system of cavities filled with fibrous calcite ( p hoto P. Playford).
,1, ,
F i gure 111-A polished core slab from
the reef facies of the Upper Devonian
Redwater Reef complex, Alberta, Canada. The rock is a framestone of massive stromatoporoids, and skeletal packstone S.W.D.W. no. 5, depth
3689 ft (photo J. L. Wray).
Figure 113-A stromatoporoid rudstone
from the reef facies in which interparti Figure 112-A po l i shed slab of l a mi n ar r cle spaces are filled with cement; Swan stromatoporoids in skeletal rudstone to Hills Re ef Complex, Beaverhill Lake For grainstone of the reef flat facies; Upper mation, Albe�ta (10-15-67-10W5); depth 7959 ft. The Interparticle spaces are fillDevonian Leduc Formation, Golden
Spike reef complex, Alberta, 10-27-51-27W4 (photo R. A. Walls and E. W. Mountjoy).
ed with cement (coin 2.1 em diameter)
(photo
R.A. Walls).
Figure 114-A porous, dolomitized skeletal sand from the reef facies; Upper Devonian Nisku Fo rmati on in West Pem bina Field, Alberta (6-25-50-1 OW5), depth of 8488 ft (coin 2.5 em diameter) (photo R. A. Walls).
Reef
'
1
�
:.l
�
401
r:
��%'!�_. .,.., ,
Figure 115-A stromatoporoid rudstone the reef facies in which the pores filled with banded radial cement and '""ns••"uent laminated internal sediment; Devonian Keg River Formation, Field, Alberta (2-27-1 08-9W6), 6529 ft (photo A. A. Walls).
Figure 116-A stromatoporoid rudstone from the reef facies in which the whole rock is dolomite and the stromatoporoids have been leached resulting in excellent moldic porosity; Upper Devonian Beaverhill Lake Formation. Kabob South Field, Alberta (1-11-59-18W5), depth 10,069 ft (photo R. A Walls).
Figure 119-A polished slab of reef (reef flat facies) composed of the stromatoporoid Stachyoides (white), the alga? Renalcis (speckled), skeletal packstone and a large cavity system fill ed with interlaminated fibrous calcite spar and pelleted lime mud; Upper Devonian of the Canning Basin, Western Australia (photo P. Playford).
limestone
re 118-A dolomitized and porous like stromatoporoid (Stachyoides) e with a mudstone matrix and lent m,oldic porosity; Upper Devo nian Beaverhill Lake Formation. South Kabob Field (1-11-59-18W5); depth 10.945 ft (photo R. A. Walls).
.;
Figure 117-A polished core slab of massive stromatoporoids and a dolomitized mudstone matrix from the reef facies; Upper Devonian Leduc For mation, Strachan Field, Alberta (10-31-37-9W5); depth 13,572 ft (photo R. A. Walls).
Figure 120-A polished slab of Stachyoids (white)-Rena/cis {speckled) reef framestone with a large cavity infill ed with banded fibrous cement and red pelleted mud, which is in turn covered with geopetal sediment and finally equant blocky spar; Upper Devonian of the Canning Basin, Western Australia (photo P. Playford).
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Reef
Fi g u re 123 (above}-An outcrop photo of a bioherm containing huge branching c olo nie s of corals; Silurian West Point Formation, G a s pe, Quebec: h a mme r for scale (photo P. Bou rq ue ). graph
Figure 124 (below)-An outcrop photo graph of a biostrome composed of rugose corals; Silurian West Point For mation, Gaspe, Quebec; pen knife 10 em in length (photo P. Barque)
Figure 121-A coral (Oisphyflum) baf flestone, dolomitized and with excellent moldic porosity; Upper Devonian Nisku Formation. West Pembina, Alb e rt a (6·25-50-10W5); depth 8520 ft (coin 2.5 em diameter) (photo A. A. Walls)
Figure 122-A polished core slab of Am· phipora (stromatoporoid) wackestone from the back-reef facies; Upper Devo nian Beaverhill Lake Fo rmati on , Swan H ills , Alberta (10·16-67-10W5); depth 7980 ft (co i n 2.1 em di ame ter ) (photo R. A. Walls).
vironments and decrease in synoptic relief upwards, finally merging with stratiform mats in upper intertidal zones above the zone of active sedi ment movement. These stromatolites continue seaward into the subtidal environment (Playford and Cockbain, 1 976) where they grow to depths of at least 3.5 m
and possibly more. They occur in pat ches for several hundred meters o ff shore. These stromatolites range from tiny, bulbous bodies developed on otherwise flat algal mats to large complex mounds. Columnar forms almost 1 m high are most common and range from conical to club shaped (Fig. 65). Some structures are
Reef
403
Fi g ure 125-A cavity in reef·flat facies limestones in which the base contains layered geopetal sediment and the ceil· ing is encrusted with pendant Renalcis; Upper Devonian of the Canning Basin, Western Australia; lens cap 5 em diameter (photo P. Copper).
Figure 126-A cut and polished slab of a toppled Stachyoides (stromatoporoid) colony encrusted with the alga? Renalcis arounc which a cavity system became fi l led with laminated pelletal lime mud and synsedimentary cement; Upper Devonian of 1he Canning Basin, Western Australia (photo P. Playford).
elongate, either parallel or perpen dicular to the shoreline. Although the living surface of the stromatolite is soft, lithification begins a fe w millimeters to centimeters below the
su rface and the interior and lower
parts of the structures are hard. Although there has been much study of Precambrian stromatolites, much less attention has been paid to their role in the formation of reefs and reef mounds. Stromatolites are
rare but do occur in Archean (more
than 2,600 m.y. old) carbonates. By
Aphebian time (2600 to 1 700 m.y. ago), stromatolites are present in a wide variety of environments and bioherms of different types can be
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Reef
Figure 127-A thin section phot ograph in plane polarized light of Renalcis, characterized by inflated chambers with microcrystalline calcite walls_ Scale 1 mm (phot o J. L. Wray) .
Figure 128-An outcrop photograph of in place bivalves {Mega/odon) in back reef deposits immediately behind the reef facies; Upper Devonian reef com plex near Brooking Springs, Western Australia (photo P. Playford).
discerned. One impressive variety of stromatolites in a platform-to-basin transition has been documented by Hoffman ( 1 974) from the Pethei Group (circa 1 800 Ma) at Great Slave Lake, North West Territories, Canada (Fig. 66). In this sequence, the edge of the platform is generally occupied by a narrow zone, the "mound and chan nel belt" similar to barrier reefs that would form later . Large elongate stromatolites with up to 3 m relief are
separated by channels filled with cross-bedded and mega-rippled coarse-grained sands and con glomerates composed of stromatolite fragments and clasts of oolitic grainstone. This belt separates plat form facies (a complex array of laminar to columnar stromatolites and intervening ooid, intraclast and oncolitic limestones) from a slope-to basin facies (lime mudstone-shale rhythmites with bedded slump brec cias and siliclastic mudstones contain-
ing poorly laminated , small, calcareous columnar stromatolites). The vertical succession of stromatolites in a shelf edge buildup from contemporaneous strata of the Goulburn Group in the Kilohigok Basin has recently been documented by Cecile and Campbell ( 1 978). Here a subtidal unit of clastic carbonates with evidence of deposition in a high energy environment is overlain by isolated stromatolite mounds general ly in the form of elongate
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405
Figure 129-Numerous stick-like stromatoporoids (Amphipora) and a single in p l ac e head of the bulbous
stromatoporoid Actinostroma the upper
part of which has been penetrated by borings; Upper Devonian Reef Complex at Geikie Gorge. Western Australia
(p�mto P. Playford).
Figure 130-An outcrop photograph of a coarse-grained pelmatozoan rudstone from the fore-reef in which individual col umals average 2 em in diameter; Silurian West Point Formation, Gaspe, Quebec (photo P. Bourque).
hemispheroids with synoptic relief of from 30 to 40 em . The main part of the buildup is composed of large mounds greater than 1 m wide and more than 1 m high that coalesce into thick laterally extensive sheets separated by intraclast rich car bonates, calcareous siltstones and sandstones. Elongate sets of mounds attain heights of greater than 10 m and have synoptic relief of up to 2 m . The upper part of the buildup is com posed of extensive sheets of laterally
linked hemispheroidal columnar and branching stromatolites. The sheets and columns are cut by narrow chan nels filled with clastic carbonate. The buildup can be traced for 1 00 m along strike and is associated with thick deposits of clastic carbonate. Early Paleozoic Reef Mounds The first metazoan reefs developed at a time of plate fragmentation, wide cratonic seas and open continental margins. During this period of
remarkable continental stability sediments were deposited in response to a gradual inundation of extensive, relatively flat cratonic areas. These shallow, Cambro-Ordovician epeiric seas were bounded by narrow con tinental margin facies of ooid/skeletal lime sand shoals; the small pioneer buildups appear to have developed in the lee of, or slightly downslope, from these shoals.
406
Reef
Figure 131-An areal view of the Lloyd Hill pl atf o rm atoll-pinnacle reef complex about 0.5 km in d ia m et er Upper Devo· nian, Western Australia, which has weathe red in relief in contrast to the sur rou n d i ng soft basinal shales (ph o to P. Playford). ,
Figure 1 32-An outcrop photograph of a typical stromatactis calcilutite; Siluri an
West Point Formation. Gaspe, Quebec; pe�cil for scale (ph_oto P. Bourque) .
.
Early Cambrian
Reefs, or more precisely reef mounds, are found in the earliest Cambrian strata, even prior to the appearance of trilobites, on the Siberian platform. These small struc tures, meters in diameter, are com posed primarily of lime mud and calcified algae (Epiphyton, Renalcis and Girvanella) while the relatively large skeletal metazoans, goblet shaped archaeocyathans, are found only as accessory elements scattered in and around the structures . Very
quickly, however , a wide variety of sessile and vagrant calcareous benthos developed on and near these sea-floor reliefs so that by the end of Early Cambrian time small but complex reef mound ecosystems exhibiting all of the sedimentological features of modern reefs, such as prolific skeletal growth, internal sedimetnation and early cementation and bioerosion, had developed. Individual bioherms and biostromes in the latest phases of Early Cam brian time are usually composed of
numerous, small stacked mounds (saccoliths), and are surrounded by skeletal sands composed of pelmato zoan debris together with brachiopod and hyolithid skeletons. Ar chaeocyathans are the most con spicuous skeletal elements of these mounds. Between these fossils are skeletons of trilobites, hyolithids, brachiopods and sponge spicules set in a matrix of lime mud. Renalcis and Epiphyton encrust archaeocyathans and line the walls of growth cavities. These cavities , which are often par-
Reef
407
Figure 133-An outcrop photograph of "Zebra-type" stromatactis which occurs sporadically in the Gros Morbe facies of the Silurian West Point Formation (Silurian), Gaspe. Quebec (pt1oto P. Bourque).
Figure 134-A peloidal lime mudstone to wackestone in which cavities are in places filled with a second generation of sediment (brown) and/or geopetal sedi ment and cement ("stromatactis"); a deep-water bioherm of Upper Devonian age, Canning Basin, Western Australia (photo P. Playford).
tially filled with fibrous synsedimen tary cements sometimes contain specific and different faunal elements from those present on the reef ex terior. At the same time, bioerosion, probably by worms, is obvious in many of the skeletons and on hard substrates. Although occasionally il lustrating two stages of development, these structures are generally homogenous.
Middle Cambrian to Lower Ordovician
With the extinction of ar chaeocyathans at the end of early Cambrian time, reef mounds lost their major skeletal elements . This seems to have had a profound affect on reef growth because most of the buildups in succeeding Cambrian strata are algal , with scattered in vertebrate skeletal elements, but much reduced in diversity compared to the early Cambrian.
Bioherms in the wide epeiric seas were mostly stromatolitic, and while some illustrate well-laminated internal structure, others show the effects of associated invertebrates whose bur rowing has destroyed the laminations. Those stromatolite-shaped structures with a non-laminated clotted, and fenestral internal fabric have been called thrombolites . The stromatolites and thrombolites are often an in tergrowth of non-calcified (blue green) algae and calcified algae, par-
408
Reef
Figure 135-A thin section of a "stromatactis" cavity. Each cavity is fill ed with bladed fibrous calcite spar (sub marine cement?) interlaminated at the base of some cavities with pelleted lime mud; Upper Devonian, Canning Basin.
Western Australia (photo P_ Playford).
Figure 136-A "neptunian dike" in a reef facies filled with red laminated peloidal marine sediment that was cemented early; Upper Devonian. Win· jana Gorge, Western Australia
(photo P. Playford).
ticularly Girvanella, Renalcis and Epiphyton. There is a suggestion that those algal buildups at or near the continental margin were composed primarily of calcified algae and abun dant fibrous synsedimentary cement while those on-shelf buildups were dominated by blue-green algae . As more and more sessile skeletal invertebrates appear in Late Cam brian and Early Ordovician time, these predominantly algal buildups are gradually populated by a more
diverse skeletal biota. Among the more important of these organisms are siliceous (lithistid) sponges , the stromatoporoid-like metazoan Pulchrilamina, the large receptaculitid alga Calathium and sometimes the primitive coral Lichenaria. Thus, bioherms in Lower Ordovician strata near the continental margin are once again largely skeletal in composition . Middle Paleozoic Structures The onset of Middle Ordovician
time coincided with a dramatic change in carbonate environments. Many continental margins were destroyed by plate collision and orogenesis while on the continents major intracratonic downwarps began to develop. Bioherm and carbonate bank development, while still present along some passive margins became widespread in the new intracratonic basins. Buildups of this age are best known from North America, Western Europe, North Africa, and Australia.
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409
Figure 137- The Muleshoe Canyon bioherm, a reef mound composed primarily of lime mudstone (Waulsortian Type) in the Lake Valley Formation (Mississippian). Sacramento Mountains, New Mexico. The structure is approx· imately 60 m thick.
Figure 138-A view of the right side of Muleshoe bioherm pictured in Figure 137, illustrating two smaller satellite bioherms each about 15 m high that grew on the flanking beds which dip to the right from the main buildup.
The acme of Paleozoic reef growth, in terms of variety of structures , geographic and paleotectonic settings, and diversity of reef forming taxa, occurred during the Siluro-Devonian . This is largely due to the appearance, at the beginning of Middle Ordovi cian time, of large sessile benthic organisms such as corals and stromatoporoids which grew in many different shapes. The reefs were built by these metazoans together with a diverse fauna of sponges, bryozoans,
calcareous red and green algae, brachiopods and pelmatozoans which increase in number and variety throughout the Silurian until latest Devonian when , coincident with ex tinction of many of the important groups, the reef ecosystem collapsed. When viewed as a whole these buildups illustrate the complete spec trum of reef types, from small reef mounds on the platform interior to extensive reef-rimmed platform atolls and pinnacles to platform-margin
barrier reef complexes to downslope mounds to basin-center pinnacles. Middle Ordovician to Late Devonian
With the varied and complex groups of organisms present , these Middle Paleozoic buildups commonly exhibit well-developed vertical and lateral ecological zonation . The reef mound, whether as an isolated structure or as the base upon which more complex reefs grew, is characteristically well developed . In
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Reef
Figure 139-A close view of the limestone that makes up the core of Muleshoe bioherm. In this instance, the fenestrate bryozoan fragments have been silicified and so stand out as brown skeletons and particles.
Figure 140-A close view of the coarse crinoid-rich limestones which make up the flankina beds of Muleshoe bioherm.
some cases, such as the Middle Or dovician of the western United States, Silurian of Scandinavia, eastern United States and Canada, and the Devonian of western Europe, these structures are carbonate mud mounds with few if any skeletal components , but abundant Stromatactis and evidence of early cementation . In other situations they contain small skeletal elements. In Ordovician and Silurian mounds these skeletons are primarily ramose and encrusting bryozoans , but appear to be replaced
by corals in the Devonian . The more complex buildups are composed mostly of stromatoporoids, and to a lesser extent, corals. The stromatoporoids are most numerous and largest and exhibit the most diverse forms in the Devonian but show a wide variety of growth forms throughout. They appear to be most prolific in the upper parts of reefs. Tabulate corals occur with the stromatoporoids and dominate the downslope environments in more tur bid water . They are relatively small in
size, but have a wide variety of growth habits, in particular such forms as the Devonian genus Alveolites which is much like the modern Montastraea. While solitary rugose corals were best adapted to life on soft substrates, colonial rugose corals attained a wide variety of mor phologies, for example the spec tacular dendroid to fasciculate Devo nian forms such as Disphyllum (up to 2 m) which resembles the later Triassic and Jurassic coral Thecosmilia.
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411
Figure 141-Massive reef limestone (right) of the Nansen Formation {Pe rmo Pennsylvanian} extendin g downward and basinward into dark argillaceous limestones of the Hare Fiord Formation (left); arrows p oin t out small reef-mounds developed on the seaward slopes of the reef front; western side of Blind Fiord, Ellesmere Island, North West Territories.
F igure 142-A small reef mound, ap proximately 30 m hi g h in the Nansen Formation (Permo-Pennsylvanian) on the eastern side of Bl ind Fiord, North West
Territories, Canada. The massive reef mound limes tones are mainly phyll oid CJ.Igae floatstones to balflestones.
Although dominated by skeletal in vertebrates, the calcareous algae which occur in the earliest Cambrian reefs are still present in these struc tures . In particular , the tiny, chambered, encrusting form Renalcis occurs as a major and diagnostic component of many shelf margin reefs . The calcareous algae Solenopora and Parachaetetes are also common, but as nodules. The flanking beds in both reefs and reef mounds are predominantly com posed of debris from pelmatozoans
(crinoids, blastoids and cystoids) and brachiopods. Crinoid flank beds are not so abundant in Devonian as Silurian buildups . Late Paleozoic-Early Mesozoic Buildups Near the end of late Devonian time, the complex reef ecosystem that gave rise to the extensive Siluro Devonian buildups collapsed . By the close of Frasnian time many of the marine invertebrates that populated the seafloor had vanished. The bulk
of the coralline skeletal elements and reef-builders were hardest hit ; stromatoporoids were reduced to a few genera, rugose corals were dramatically altered, and tabulate corals became extinct. Brachiopods were decimated , and although they recovered in the later Paleozoic they were never again as diverse. The following early Mississippian had few if any "reef-building" taxa, with the place of the corals and stromatoporoids being taken by pelmatozoans and bryozoans.
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Reef
Figure 143- The margin of a reef mound in the Late Pennsylvanian Holder Formation, Dry Cany on Sacramento Mountains, New Mex:co. The structure, so me 8 m in height, is composed of phylloid algae and lime mud. ,
Figure 144-The flanking
beds,
in an
outcrop some 12 m thick. of a large reef mound exposed in the Upper Pennsylva nian Holder Formation in the walls
Dry Canyon, Sacramento
of
Mo unta in s , New Mexico. These beds, with a steep westward dip (to the left) drape off a mound which is out of the p la n e of the
photograph
and is composed of algal plates and pelmatozoan debris (photo 8. Pratt).
Gradually though, during the ensuing Mississippian and early Pennsyl vanian, a variety of new carbonate producing organisms appeared . These generally small organisms evolved continuously through early Permian time into the late Triassic. Among the most important of these new taxa are phylloid calcareous algae (Ar chaeolithophy/lum, Eugoniophyllum
and Ivanovia), and Tubiphytes, an enigmatic calcareous organism which grew as tiny, laminated epiphytic en crustations in reefs from Early
Mississippian to Late Jurassic, but were principal components only in Permian, Triassic and some Jurassic buildups. Other important forms are Opthalmidid-Calcitornellid tubular foraminifrs, small dendroid stromatoporoids (for example, Komia) and the encrusting coralline alga? Archeolithopore/la. Calcareous sponges also become a conspicuous part of the reef biota in Permian time as do spongiomorph hydrozoans (stromatoporoids) in the Triassic. The first large, massive frame-building
organisms, scleractinian corals (the reef builders of modern seas) appear in the Middle Triassic and together with calcareous sponges and stromatoporoids alter the make up of buildups dramatically from reef mounds to true reefs. Most Late Paleozoic buildups occur in and around the margins of in tracratonic basins. In Permian time, with the breakup of Pangea, a large circumglobal seaway, the Tethys, gradually developed and the majority of subsequent Mesozoic reef growth took place along the margin of or in basins adjacent to this ocean. Mississippian
Although the reef ecosystem col lapsed during Devonian time, bioherms of Mississippian age are often substantial structures, rising as much as 1 50 m above the sea floor and having slope deposits dipping as much as 50°. They are found chiefly near shelf margins and in deep water
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413
Figure 145-A d etail ed outcrop photograph of phyllold algae bafflestone in the Gobbler Formation, Upper Penn sylva nia n , Sacramento Mountains, New Mexico. The alga is a lmost all the genus Archaeolithophy/lum and the matrix be· tween the pl ates is ske l et al lime wackestone.
Figure 146-A cut and pol ished slab of phylloid alga-hydrozoan bafflestone from the core of a Lower Permian reef mound i n the Nansen Formation, Ellesmere Island, North West Territories, Canada. Most of the large skeletal plates here are of the hydrozoan Palaeoaplysina, and each is su rrounded by th ick rinds of fibrous calcite cement (photo G. Davies).
Figure 147-A thin section photograph of Fenestellid bryozoan (lines of small dots) f lo atston e in which the bryozoa are surrounded by thick rinds of cement and the voids are filled with skeletal packstone. from the Nansen Formation, Pennsylvanian, Ellesmere Island, Norlh West Territ or ies , Canada (scale 1 em) (photo G. Davies).
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Reef
Figure 148-An outcrop photograph of byrozoan framestone with boytryoidal calcite cements from reef mounds in the Nansen Formation, Lower Permian,
Ellesmere Island, North West Territories, Canada.
Figure 149-A cavity filled with laminated, dolomitized sediment in a Pennsylvanian bryozoan reef mound in the Nansen Formation, Ellesmere Island, North West Territories, Canada.
structure itself. In late Mississippian time, isolated corals became locally important con stituents in these bioherms. Pennsylvanian
settings. The scarcity of large skeletal biota is dramatically illustrated in the style of these buildups. They are primarily lens-like mounds composed 50 to 800fo massive, vaguely clotted, peloid lime mudstone with only scattered fragments of crinoids and bryozoans. These structures have come to be call ed "Waulsortian mud mounds" after a locality in Belgium. The mud is often multigeneration with later phases filling cracks as geopetal sedi ment in what must have been partial-
By mid-Pennsylvanian time, the newly evolving calcareous benthos had begun to populate the mud mounds, and structures of this age contain phylloid algae as a major component. These buildups are somewhat smaller, on the average, than their Mississippian counterparts, generally up to 30 m high with ly lithified reef rock. Original cavities skeletal flanking beds dipping off the are also filled with laminated sedi cores at about 25°. The phylloid algae with their carbonate-encrusted ment and fibrous cement forming multiple , regular and parallel leaves probably grew upright in Stromatactis structures. Extensive ear thickets and gardens on and around the reliefs on the sea floor . In some ly lithification is also illustrated by instances, encrusting bryozoans and occasional fragments of mound rock containing Stromatactis in flank beds. foraminifers grew on the mounds as In European examples, flanking beds well and these were often as abundant are present but not extensive, while in as the algae. North American examples, the flank Although most of the buildups are ing beds made up of coarse encrinite small reef-mounds, in the subsurface and rare lithoclasts are thick, at times these buildups form a major compo comprising more than half of the nent of what were large offshore
Reef
415
Figure 1 50- Botryoidal calcite in reef mounds of Pennsylvanian age from the Nansen Formation, Ellesmere Island, North West Territories, Canada. On the basis of similar mic rofabrics, geometry and high stronti um content (8000 ppm) these are interpreted to be neomorphos· ed botryo idal aragonite, prec ipi t ated within the reef m o und as it grew on the sea floor (photo G. Davies}.
Fig u re 1 5 1 -Looking north east toward El Capitan, a massive ca rbonate that forms the western ma rgi n of the Per m i a n reef complex in the Guadalupe Mounta ins of west Texas and New Mex ico. In t h is photog raph , the massive cliff form in g li mestones at the center and left are the olde r fore-reef facies of the u nder l yi ng Goat Seep reef. The massive l imesto ne at the right is the plat f orm margi n , Capitan reef to fore-reef facies of t h is co mplex .
banks growing in deep basins. Here they are characteristically zoned with red algae, fusilinids, algal calcispheres and crinoids most common on the oceanward margins, and green algae, rugose corals and brachiopo ds usual on the leeward sides. Permian
Buildups in earliest Permian strata are still predominantly phylloid algal mounds but these plants are soon augmented by a variety of low-lying, delicate, branching to encrusting
organisms such as sponges,
Tubiphytes, and the encrusting form
Archaeo/ithoporella (which may be an
alga or an inorganic precipitate), together with bryozoans and other calcareous algae so that most Per mian bioherms are dominated by this new biota.
Perhaps the most famous of these buildups is the "Permian Reef Com plex" in the Guadalupe Mountains of west Texas and new Mexico, southwest United States. This uplifted western margin of the Delaware Basin exposes an extensive carbonate plat form complex which in the subsurface to the east is one of the world's great oil-producing structures. These rocks have been described in several major works (see Wilson, 1975 , for an ex cellent summary). The marginal facies of this complex has long been an enigma because the massive unit which forms such an impressive
"rim" at the edge of the shelf is a muddy carbonate with abundant fine debris and small encrusting organisms and in many places an almost com plete absence of relatively large frame-building taxa. It is, therefore, difficult to imagine it as a barrier reef similar to modern structures even though the location is right . Rather, it now appears that the original high point on the marginal profile was a lime sand barrier island complex and the "reefy" facies rich in sponges , Tubiphytes and cement-lined cavities is a down-slope deposit of coalesced reef mounds.
416
Reef
Figure 1 52- Looki ng south paral lel to t he m a r gin of the Permian Reef Complex in McKittr ick Canyon that runs normal to the t re nd of t h e complex I n t h e u ppe r part of the wall on the skyl i ne is the tran sit i on from bedded back-reef sediments (lett) to the massive platform margin or " reef" carbonates (center) to the forereef strata ( r i gh t) that dip ba s i nwa rd . The in c l ination of beds from upper left to lower r i gh t in the ca n yo n wall i s the dip o f older fore-reef s t r ata.
F i g u re 1 53-An outcrop photograph of an incl ined surface in the massive " reefal " portion of the Permian Reef Co mpl ex near the m ou th of Walnut Can·
yon, New Mexic o . The limestone here is a framestone co mpos ed of sponges and
Tubiphytes
i n a skel etal wackestone
matrix (hammer blade for scale).
The only skeletons larger than a few centimeters in these reef mounds are the conspicuous calcareous sponges, especially the sphinctozoan or chambered forms. The matrix of lime mud makes up over half of the rocks while the other biota present are encrusting algae and/ or foraminifers , Tubiphytes and locally, phylloid algae. Most of the growth cavities are filled with spectacular growths of fibrous cement which was probably marine. Some earlier Per mian mounds in this area appear to
be over one-half marine cement . In other regions, such as Tunisia, Sicily and eastern Asia, calcareous sponges are also the primary com ponents of Permian reef mounds. In eastern England, however , the Magnesian Limestone, which also forms an extensive shelf-margin com plex, appears to contain mostly bryo zoans and abundant encrustations identical to Archaeolithoporella. Likewise, lens-shaped reef mounds in the southern Alps, which reach thicknesses of over 300 m in
downslope faces are dominated by Tubiphytes, Archaeolithoporella and bryozoans and contain abundant synsedimentary cement, but lack sponges. Triassic
Reefs of Triassic age are developed mostly along the northern and southern margins of the widening Tethys and along the western coast of North America. These reefs are best studied in the Alps where they occur on and marginal to huge carbonate
Reef
417
Figure 1 54-An outcrop photograph of an i n c l i ned s u rf ace i n the massive " reefa l " portion of the Permian Reef complex near the mouth of Walnut Can yon, New Mexico, in which sp onge s and other skeletons are bound by numerous encrustations (i rreg ular dark l i ne s) of the calcareous ulga Archaeolithoporella.
Fig u r e 1 55-An o ut c r op photograph of th e massive reef facies of the Permia n Reef Compl ex exposed in D a r k Canyon. Here a pocket of s keletons ric h in sponges and large gastropods occurs in an otherwise massive wackesto n e to mudstone ( h a mm e r blade for scale at top).
platforms which are similar to those of the preceding Permian. There appears to be no record of early Triassic reefs and a scarcity of reef-building biota world-wide. The first Triassic reefs, of mid-Triassic age, are small deep-water buildups with sparse corals and echinoderms . These are followed by extensive reef complexes in late Middle Triassic strata in which the most important reef-building biota are calcisponges , Tubiphytes, some stromatoporoids and many colonial corals; essentially the same faunal elements as in the preceding Permian, but with the im portant addition of colonial corals. Although most reefs contain this varied biota some are dominated by Tubiphytes. The most extensive car bonate platforms developed during the late Triassic (Norian-Rhaetian) with marginal facies composed of sand shoals and reef complexes. Many of these platforms, however, were destroyed during the latest
Triassic and transformed into basins with terrigenous sediment and areas of small reefs . Those structures studied to date along the west coast of North America appear to be small patch reefs without ecological zonation and may be deep-water , ahermatypic buildups. The main biotic constituents of Up per Triassic reefs are corals , stromatoporoids (hydrozoans), calcisponges and calcareous algae.
The corals and calcisponges are the most important components but are restricted to different parts of the reefs and different zones of water energy; corals in shallow, high energy environments (especially the bran ching form Thecosmilia); calcisponges in the more protected and sheltered central parts of the reef. Colonial corals in these structures exhibit the same wide range in growth form as seen in modern reefs. Stroma toporoids are also abundant and oc-
418
Reef
F i gure 1 56-The back- reef fa c ies of the Permian Reef Complex (Tansill Forma tion) in Dark Canyon i l lu strating a large overturned colony of Collenella (hammer
blade for scale).
Figure 1 57-An outcrop photograph of an i n cl in ed surface in the massive " reefal " portion of the Permian Reef Complex near the mouth of Walnut Ca nyon, New Mexico. This part of the out crop contains many spherul ites or fan shaped growths of calcite which were probably precipitated as aragonite into ope n spaces of the sea floor w h i le the limestone was fo rm i ng (pencil for sc a le) .
Figure 1 58-The 200 m thick Steinplatte Reef of U pper Triassic age near Waid· ring i n Tirol in which the massive reef limestone (right) passes into the fore-reef slope (left) dipping at about 30" into the Kessen basin (photo H. Zankl).
Reef
419
Figure 1 59- The massive l im estone which comprises the core of the Dachs teinkalk Reef, Upper Tri ass ic (Norian) in
the Gosaukamm Range, E Fl u ge l).
Austria (photo
Fig u re 160-The m ature or diversifica tion st ag e of Upper Rhaetian patch reefs at Tropfb ru ch near Halle i m , Salzbu rg , Aust ria . here i l l ustrating abundant cor als; the large bran ching coral in the ce nte r is 0.5 m high (photo E . F l u gel).
cupy a variety of different en vironments, mostly in protected parts of the reef. Together with these, bryozoans and calcareous algae con tribute to the bulk of the reef mass. The major sediment producing organisms are brachiopods, bivalves, gastropods, cephalopods, serpulid worms, crustaceans, ostracodes , crinoids, ophiroids and holothurians. These buildups exhibit, for the first time, extensive bioerosion by bivalves and algae. Although boring organisms are known from the earliest reefs, these are the first struc tures in which they play a significant role as a diagenetic agent in reef development. As in the Permian many of the growth cavities are oc cluded by extensive fibrous spar inter preted to be synsedimentary cement . These late Triassic structures, like those in the middle Paleozoic , exhibit the complete spectrum of reef growth from on-shelf reef mounds and patch reefs , to marginal reefs with excellent lateral zonation, to deep-water reef mounds. Many of the reefs have the characteristic four stage development
but are commonly "drowned" after reaching the domination or climax phase. Jurassic Reefs, although known from the Lias of Morocco, only become a common element of Tethyan car bonate facies in Middle Jurassic time, reaching a maximum in Late Jurassic. They occur both as patch reefs in shallow marine basins in western Europe and as marginal facies around the northern edge of the Tethys from
eastern Canada to the Middle East and along the rims of large but isolated platforms within the Tethys. The structures themselves appear to be mostly isolated deep water reef mounds or numerous small patch reefs and reef knolls. Deep water reef mounds, which may have relief of up to 1 00 m , are composed of siliceous sponges , algae and tubular foraminifera set in a lime mud matrix . The most conspicuous associated fauna in these structures are bryozoans and brachiopods.
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Reef
Fig u re 1 6 1 -The same loca l ity as il lust rated i n Figure 1 60 but here the l imestone has suffered partial disso l u tion and the holes are filled with geopetal sediment and white calcite spar; divi sions on the meter stick are 20 em (photo E . Flugel).
Fi g u re 1 62-The same local ity as above i l l ustrating a ha rdground su rface en crusted by a sp h i n c tozoa n sponge which is about 2 e m th ick. The subhorizontal l ines across the photo are saw-marks on the quarry wall (photo E. Flugel).
Coral/stromatoporoid reefs grew as patches on wide shallow platforms, as belts of patch reefs near shelf margins, and as caps on pre-existing sponge reef mounds. In some areas these reefs are coral-dominated and exhibit excellent vertical zonation , while i n other regions they are thinner with the principal elements being stromatoporoids and the encrusting coral Microsolena. The red calcareous algae Sofenopora as well as the green Dasycladacean algae reach the height
of their development in Late Jurassic time. In addition, the modern types of Codiacean algae and articulated Corallinacea first appear associated with these buildups . Bioerosion is conspicuous in these structures , with sponges now being one of the prin cipal endolithic taxa together with algae and bivalves. The marginal patch reefs are often up to 8007o skeletal debris from the remains of crinoids, snails, solitary corals and large bivalves .
Cretaceous
The late Jurassic reef-building com munity persisted into early Cretaceous time with shelf-margin and ramp buildups composed primarily of the coral-algal-stromatoporoid communi ty. At the same time one group of mollusks, the rudist bivalves, evolved rapidly and by mid-Cretaceous time were important biotic constituents of most buildups. These bizzare bivalves usually had one large valve attached to the substrate while the other
Reef
smaller valve was a lid or cap. They adapted to a wide variety of habitats from muddy lagoons to platform margins to foreslopes and in these different environments developed shapes that varied from prolate and planispiral (Requinids) to clumps of tall, twisted and slender (Caprinids) to massive and keg-shaped (Radiolotids) individuals with some shells as large as 1 .5 m in length. In shape they closely resemble ar chaeocyathans, rugose corals and richtofenid brachiopods. During middle Cretaceous time (Aptian through Cenomanian) the rudists diversified into several reef and back-reef habitats and locally in the surf zone. Mid-Cretaceous reefs grew at the margin of broad shelves and on the inner shelf, often in areas of restricted circulation. The high diversity shelf-margin reefs consisted of a core community of corals, algae and a few rudists. The rudist com munities tended to occupy the shallow, high-energy reef flat zone. Inner-shelf, low-diversity reefs were dominated by one or a few species of rudists that tended to form biostromes, patch reefs or high energy shoreface beds marginal to high salinity lagoons or sabkhas. Upper Cretaceous buildups are dominated by rudists, particularly radiolitids, with corals and encrusting
Figure 1 64-A t h i n section photomicrograph of typical Upp e r Rhae· tian reef l imesto ne i l l ustrating calc isponges (circ u l a r c ross-sections) and encrusting foramin ifers (dark i r· regular mosaic between sponges); in dividual sponge cross sections average 75 mm in diameter (photo E. Flugel).
algae only minor constituents. On shelf buildups were generally biostromal while patch reefs developed near the shelf margin. Cor als apparently diminished as frame builders because of secular en vironmental changes , perhaps in salinity, temperature or oxygen con tent. Competition by rudists does not seem likely because in Albian time corals and rudists occupied different environments. Cenozoic Reefs The catastrophic mass extinctions at the end of the Cretaceous, while affecting benthic calcareous taxa dramatically, did not wipe them out completely. Rudist bivalves disap peared completely while colonial cor als survived but were greatly reduced in diversity from about 90 to 30 genera. Calcisponges and stromatoporoids (spongiomorph hydrozoans) also survived, but the en-
421
during forms were few in number and never again important as reef-building taxa. Cenozoic reefs are essentially the same as modern reefs, dominated by scleractinian corals. Their record of development, however, is not well known, largely because ( I ) in tec tonically stable areas they are still buried while in tectonically active areas the facies relationships are obscured by faulting, and (2) they oc cur in the tropics where poor out crops are altered by deep weathering. The distribution of Cenozoic reefs in time and space is a direct result of changes in oceanic circulation pat terns brought about by the pro gressive blockage of the Tethyan seaway by plate interaction. Although there are reefs of Paleocene age, most of the corals in them are survivors from the Cretaceous. Solenopora, an important coralline alga in reef growth since the Ordovician, disap pears at the end of the Paleocene. Beginning in the Eocene there was a new radiation of hermatypic coral genera. It seems that these were spread throughout the diminishing Tethys because the Northwest Indian
422
Reef
Fig ure 1 65-A negative peel of an Up per Triassic D a c hstein Reef limestone. The branching coral Thecosmi/ia togeth e r with oth e r corals and ca lc a reo us sponges forms the framework. E ncrusting forami1ifera ( l ight rims) and a l g a l c r u sts "Spongiosiromata" ( l o we r left) su rround the co rals and II e the i n te r n a l cavities. Fine geopet a l sed im e nt is found with in and between .he corals and sponges {scal e 1 em) (photo H. Za n k l) .
Figure 1 66-Th e ridge on the skyline is com posed of a series of small bioherms
with each pi n n acle bei ng the massive
resistant core of a reef. 30 to 50 m thick. The section of
Middle Jurassic strata
exposed here at Jebel Assameur n' Ait
Fergane . Morroco. is about 200 m thick. The recessive slopes are fine-grained marls and limestones (photo J.
Warm e) .
Ocean was still connected to the Mediterranean across a shallow seaway in the Middle East. This evolution continued through into the Oligocene at which time Cenozoic reefs reached the height of their development, in terms of range of reef communities , luxuriance of growth and numbers of buildups reported . The major change in reef distribution took place during the Miocene with the formation of the Mediterranean as a separate sea and the appearance of polar ice caps . By
the end of Miocene time there were no coral reefs left in the Mediterra nean. During ensuing Pliocene time the compression of climatic belts together with the rise of the Isthmus of Panama restricted reef growth to two regions, the Indo-Pacific and the Caribbean , the sites of modern reef growth. Oligocene-Miocene Reefs During the acme of Cenozoic reef growth, the Oligocene, reefs of all types can be recognized and in most
Reef
423
Figure 167-A partially e r oded bioherm core (center) and inclined off-reef strata in the M iddle J u rassic of the High Atlas. Morocco. The core is composed ol
scleractlnlan corals and skeletal
wackestone while the flanking beds are mainl y coral debris (photo J . Warme).
Figure 1 68-Three off-lapping Lower J urassic scleracti nian coral reefs in the gorge of the river Ziz, High Atlas Moun tains. Morocco. The reef core i s at center right 1 2 m high. and inclined off-reef strata are at center left (photo by J. Warme).
cases a vertical as well as horizontal zonation discerned . The early stages of reef growth are characterized by low diversity assemblages of corals with light, porous, quickly-growing skeletons that could reject sediment and colonize muddy substrates; for example, Goniopora, a branching to encrusting form and Actinacis with superposed branches and plates like the modern Acropora palmata. The diversification stage of growth is
characterized by great numbers of species, but as in modern Caribbean reefs where seven of the 42 species dominate the reef framework, of the 30 Oligocene genera massive Goniopora and Favia as well as Mon tastraea, Dip/aria, Pavona, Col pophyllia and A ntiguastrea form most of the reef frame. Other abun dant forms such as the rapidly grow ing branching forms Acropora (or Dendracis), Actinacis, Goniopora,
Dictyaraea and Stylocoenia and the nodular forms A/veopora and Astreopora are also found in great luxuriance. Other reef-dwelling fauna such as coralline algae, foraminifers (especially encrusting forms) and bryozoans are similar to those in modern reefs . A major difference, however, is the relative scarcity of skeletal plates from the Codiacean alga Halimeda. These particles, which comprise most of the skeletal sand
424
Reef
Figure 1 69-An outcrop ph otog rap h of the co re of one of the bioherms il lustrated in Figure 1 66 composed of a m ass i ve sc leractinian co ral with an erod ed upper portion a n d several in-place boring bivalves (center left); pen cap is 6 em i n lengtt1.
Figure 1 70-A massive bran ch i n g coral from the core of one of the bioherms il lustrated i n Figure 1 66 (photo J. Warme).
produced on modem reefs, appear to be relatively rare in pre- mid-Miocene reefs. The Miocene reefs of Indonesia il lustrate similar complexity but those in the late Miocene of the Mediterra nean reflect the gradual onset of con ditions inimical to growth . These Messinian structures are generally narrow fringes around the steep margins of islands. Most Miocene reefs of the region �pntain a wide variety of species ( 5 to 1 5) with such forms as Tabellastraea, Porites, and
Montastraea as well as other reef dwelling biota including echinoids, bivalves , gastropods, barnacles, foraminifers, bryozoans and coralline algae. Those of late Miocene age are built almost exclusively by Porites, in long vertical sticks up to 4 m high and 2 to 3 em in diameter, together with some bryozoans, coralline algae and serpulid worms entombed in marine cement. The extensive fore reef deposits are mostly coralline algae and Halimeda. This dominance of one genera in the late Miocene
likely reflects the first incursions of now cold Atlantic waters into the western Mediterranean and foretells the disappearance of reefs from this region . One o f the most spectacular at tributes of modern reefs is the reef crest zone, which is dominated by rapidly-growing species of Acroporidae, Poritidae and Seriatoporidae and which is accreting at rates of up to 1 0 to 1 5 m every 1000 years. These species are not present in pre-Pleistocene structures ,
Reef
Figure
1 7 1 -A
425
close view ot a r11assive
b ranching sc lerac ti nian coral from the
flanks of one of the bioherms illustr ated
i n Fig u re 1 66 (photo J . Warme).
Figure 1 72- The basal portion of a mid dle Jurassic scleract in ian cor al bioherm in the H igh Atlas M o u n ta in s of Morocco. Th e rock is a bafflestone to floatstone of branching and stick-like corals; cap of pen is 5 em i n length (photo J. Warme).
and Acropora, for example, although found in Eocene-Pliocene coral com munities is rarely abundant , not thickly branched as in modern struc tures , and localized to quiet en vironments . As a result, Cenozoic reef accretion rates are much lower, more in the order of I to 3 m every 1000 years. Cycles of Reef Growth When the maj or trends in reef growth together with the variation in reef-building taxa are set against
geologic time, and the short-term variations ignored, two major cycles of reef development seem to emerge, an early short CYCLE I of about 240 m.y., and a later longer CYCLE II of about 340 m.y. (Fig. 204). The general aspects of each of these cycles are the same, an early period in which only a few reef-building taxa are present , mainly small branching or encrusting organisms which form reef mounds, and a late period in which large skeletal metazoans become important and construct com-
plex reefs of all types. Superimposed on these general trends are two ab berant episodes of reef growth , one in the early Cambrian when ar chaeocyathans were the principal elements of reef mounds and one in the middle to late Cretaceous when rudist bivalves were among the main biotic constituents of reefs and reef mounds . In the early part of Cycle I , calcified algae as tiny, branching , shrub-like clusters and anastomosing crustose masses (Epiphyton, Gir-
426
Reel
F igure 173-The core of a small bioherm from the M iddle Jurassic of Morocco. composed of numerous small stick-like corals, often bored, in a matrix of lime m ud (photo J . Warme).
complex reefs . In Cycle I I , bryozoans are present throughout, alone as the principal elements of reef mounds at the beginning and then as reef dwellers, but rarely as reef builders . The major period o f reef growth in each cycle occurs when the consor tium of corals and stromatoporoids appears . In the wholly Paleozoic Cy cle I stromatoporoids tend to dominate reefs in the later phases while tabulate and colonial rugose contls are important , but not primary elements . In the Mesozoic part of Cy cle I I , scleractinian corals are most often the main reef elements together with spongiomorph hydrozoans or stromatoporoids and calcareous sponges as important but rarely domi nant elements. Following the late Cretaceous mass extinction event, reefs are mostly built by scleractinian corals.
Fi g ure 1 7 4-A negative thin section print of an I ntensi vely bi o eroded coral
from the M iddle J u rassic of the Higll Atlas Mountains, Morocco. The white areas are lime mudstone and the curved shells within the w hite mud are the valves of the boring pelecypods that ex cavated the peel holes; photo 5 em across (photo by J. Warme).
vane/la, Renalcis) are the most essen tial elements of reef mounds. They continue to be important until the end of Devonian time. Reefs in the early parts of Cycle II are often dominated by the small encrusting organism Tubiphytes as well as en crusting foraminifers which remain fundamental components of the reef ecosystem through to the present day, even though the forms have changed . The buildups in the Paleozoic part of Cycle II also contain phylloid algae as conspicuous elements . In both cycles the major change in reef biota is heralded by the ap-
pearance of sponges in abundance, siliceous sponges in the Early Ordovi cian of Cycle I and calcareous sponges in the Permian of Cycle II. These elements at first form buildups with other small , diminutive taxa and later are either accessory to dominant elements in more complex reefs or form bioherms on their own when the larger reef biota are excluded by en vironmental constraints. Bryozoans do not appear until mid way through Cycle I when they dominate bioherms for a short time and from then on are accessory elements except in the early stages of
REEFS AND CARBONATE PLATFORM GEOMETRY
In their synthesis of modern car bonate sediments on continental shelves, Ginsburg and James ( 1 974) recognized two intergrading categories of shelves or platforms : ( 1 ) open shelves , those in which the outer shelf to slope break is in tens of meters of water ; and (2) rimmed shelves, those which have a continuous or semi continuous barrier along the shelf margin. At times when a full spec trum of reef-building organisms is present, both types of shelves may
Reef
427
Figure 1 75-A reef mound of Upper Cretaceous age (Campanian) exposed at Djebel Mokta, Tun i s i a . The mound is rich in the giant rudist Hippurites lapourizi (photo S. Frost)
Fig u re 1 76-The external molds of H. lapourizi in a reef mound simila r to that in F i gur e 1 75 at Jebel Merieg, Tunisia.
form and the margin on rimmed shelves may be either a barrier reef or a series of carbonate sand shoals or both. The most complex type of platform at such times will be a reef-rimmed
Figure 177-Radiolitid rudist bivalves from the reef margin at the eastern edge of the Valles, San Luis Poto si platform i n t h e Taninul fac ies of the El Abra the lee of the barrier reef range from Limestone (Albian-Cenomanian), Laguna circular to elliptical to irregular in Colorada, Quereta ro, Mexico; small plan and are sometimes large enough black divisions on scale in em (photo P . Enos).
platform (Fig . 202). The barrier itself is well-zoned if the front is steep and wave action intense, but zonation is weak if the front slopes gradually seaward and the seas are relatively quiet. Patch reefs on the platform in
to enclose a lagoon themselves . Reef mounds occur on the inner, shallow parts of the platform, in areas of nor mal salinity but turbid water. Reef mounds also occur at depth, in front of the barrier reef, down on the reef front or forereef. Patch reefs or reef mounds com monly form a very widespread lithofacies compared to the barrier
428
Reef
Fi g u re 1 78-A cl uster of r ad i ol i t i d rudists from the same reef complex as i l l u st rated in Fig u re 1 7 7 forming a re ef tramestone (photo P. Enos).
Figure 1 79-A " bo u q u et ' ' of radiol itid rudists from the bi o h e rm i l l ust rated i n F i g u re 1 75. The s e skeletons have top pled from their growth position and are now upside down (ph oto S F rost ) .
Figure 180-Radiol itid rudist bivalves i n an arborescent cluster f ro m t h e Tan inul facies of the El Abra Limestone (AlbianCenomanian). Mexico (photo P . Enos).
reef. The stratigraphic thickness of these reefs is dependent on the rate of subsidence - if subsidence rate is low, reefs are thin; if subsidence rate is high, reefs may be spectacular in their thickness. At times when large skeletal metazoa are absent, the small and delicate organisms cannot live in the turbulent shallow water at the shelf edge. As a result, the margin of the shelf or platform is normally a com-
plex of oolitic or skeletal (generally crinoidal) sand shoals and islands (Fig . 203) . The only reef structures are reef mounds which occur below the zone of active waves down on the seaward slopes of the shelf or plat form , and if conditions are relatively tranquil, behind the barrier, on the shelf itself.
SUMMARY
When documenting bioherms and biostromes one must not lose sight of the fact that they are primarily biological in origin. These structures are the fossil record of a community of organisms which, by continued growth in one spot, formed a sedimentary unit. Because of this, one reef is likely to be quite different from another , even in the same for-
Reef
429
Figure 1 81 --Caprinid rudist b i va l ves fill ed with coarse calcite spar from the Cerro Angel reef zone, El Doctor Plat form, Queretaro, Mexico (Albian Cenomanian); scale 1 0 em in length (photo P. Enos).
mation. Consequently, the themes outlined in this synthesis should be viewed not as a rigid classification scheme into which all reefs must somehow fit, but rather as a guide to the study of these fascinating struc tures.
ACKNOWLEDGMENTS
Figure 1 82-Large caprinid rudist bivalves in the Taninul facies of the El
Abra Limestone (Albian-Cenomanian), Mexico (photo P. Enos).
Many of the concepts on the deposition and diagenesis of modern reefs and reef like structures presented in this article were developed while at the Comparative Sedimentology Laboratory, University of Miami, with Robert N . Ginsburg. Much of the data that has allowed me to find the threads that bind reefs of different ages together is documented in the scholarly text on "Carbonate Facies in Geologic History," by James L. Wilson. This paper could not have been written without the generous co-operation of the following colleagues who gave me photographs to use and/or spent time discussing the vagaries of reef growth through geologic time: P. A. Bourque, P . Copper , G . Davies, P . Enos, M . Esteban, E. Flugel, S. Frost, P. Hoffman, D. R. Kobluk, C. F. Klappa, C. St. G . C. Ken dall, R. Louckes, R. Lighty, M . Longman, I . G . Mcllreath, E . W . Mount joy, A. C. Neum ann, P . Opalinski, B. R. Pratt, P. E. Playford, L. C. Pray, M. Rees , J. K. Rigby, A. J. Rowell , S. Rowland , R . J. Ross, P . A . Scholle,
Figure 1 83-A core from a deep well in southern Florida composed of dolomi tized Aptian caprinid rudists and wacke stone matrix. Dolomitization has substantially increased matrix porosity and permeability (photo S . Frost).
430
Reef
Figure 184-A bioherm of Upper Miocene age (M essin ian) near Almeria, Spain. The top of the hill i s a reef com posed predominan tly of Porites. Th is structure developed during a g radual sea level fall and so lies upon basinal muds and calcisi ltites (photo M. Est eba n).
Figu re 1 85-Santa Pol a reef of Upper Miocene age (Messi nian) near Alicante, Spai n. The present day topo· graphy reflects the original reef topography with the massive li mestone wall being the original reef wall composed mai nly of Porites (photo M . Esteban).
R. W. Scott, C. W. Stearn , R. K . Stevens , D. F. Toomy, R . A. Walls, J . Warme, J . L . Wray and H . Zankl. R. W . Scott kindly provided m e with data o n the nature of Cretaceous buildups through time, which I used in the preceding text. The manuscript was written during the tenure of an Industrial Fellowship at the Denver Research Center of Marathon Oil Company. The diagrams were kindly drafted at the Research Center by Richard Nervig , Barbara Steele and Arzell Thomp son . Many of the photographs were skillfully transformed from slides into
prints by Wilf Marsh of Memorial Univer sity. My own recent work on fossil reefs has been generously funded by the Na tional Science and Engineering Research Council of Canada. Parts of this article and several of the figures have already been published by the Geological Associa tion of Canada and I am very grateful for permission to reproduce them here. Final ly, this section is much enhanced by the three examples of reef reservoirs described by Dick Walls, Phil Choquette and Don Bebout, and I thank them for undertaking this endeavor.
SELECTED REFERENCES Classification and Nomenclature Cummings, 1932, Reefs or bioherms?: Geol . Soc. America Bull . , v. 43, p. 33 1-352. Dunham, R. J., 1 962, Classification of carbonate rocks according to deposi tional texture, in W. E. Ham, ed . , Classification o f carbonate rocks: AAPG Mem. 1 , p. 108-122. Embry, A. F., and J . E . Klovan, 1 97 1 , A Late Devonian reef tract on north eastern Banks island, N . W.T . : Bull.
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431
Figu·re 1 86-The spu r and g roove strUC· ture of the reef front in an Upper Miocene reef at Cap Blanc, Llucmajor, Mallorca. The spurs (arrows) are con structed by sticks and dishes of Porites; the man is standing in groove (photo M . Esteban).
Figure 1 87--A close view of the Porites reef-wall framework (plan view) in which the coral sticks are leached (diameter 1 to 2 em) and filled with red volcanic sand; Cabo de Gala, southeastern Spain (photo M. Esteban).
Canadian Petroleum Geology , v. 1 9 , p .
730-78 1 . Folk, R . L . , 1 962, Spectral subdivision of limestone types, in W. E. Ham, ed . , Classification o f carbonate rocks:
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sediments and their diagenesis: Amster dam, Elsevier Sci . Pub . , 658 p . Darwin , C . , 1 842, Structure and distribu tion of coral reefs: repr. by Univ. of Calif. Press, from 1 85 1 ed . , 2 1 4 p . Frost , S. H . , M . P . Weiss , and J . B. Saunders, 1 977, Reefs and related car
Brineman, 1962, Skeletal limestone
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AAPG Stud . in Geology, No. 4, 421 p .
Classification of carbonate rocks: AAPG Mem. I , p. 224-253 . General Articles Reviewing Modern Reefs Bathurst, R. G. C . , 1 975, Carbonate
Ginsburg, R. N . , and N . P . James, 1 974, Spectrum of Holocene reef-building communities in the western Atlantic, in A. M. Ziegler et al , eds . , Principles of benthic community analysis (notes for a
short course): Univ. of Miami, Fisher Island Station, p. 7 . 1 -7.22 . Stoddart, D . R . , 1 969, Ecology and mor phology of recent coral reefs: Bioi. Rev . , v. 44, p. 433-498.
Taylor, D. E. , ed . , 1 977, Proceedings of Third International Coral Reef Sym posium, Miami Florida; 2. Geology: Miami Beach, Fla., Fisher Island Sta tion, 628 p . Different Types o f Modem Reefs
Pacific Atolls
Emery, K. 0., J. I. Tracey, and H . S.
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Fig u re 1 88-Upward-growing masses of Porites from the re ef-wall faci es, Cabezo Maria Reef, Fortuna, southeastern Spain {photo M. Esteban).
Fi g ure 1 89-N umerous, plate-like col onies of Tarbellastraea from the deep reef-front facies In a Serravallian Lang h ian coral reef from Oierdola (Barcelona), Spain (photo M . Esteban).
Ladd, 1954, Geology of Bikini and nearby atolls: U.S. Geol. Survey, Prof. Paper 260-A, 265 p .
Marginal Reefs Gareau , T. F., 1 95 9, The ecology of
Jamaican coral reefs ; I. Species, com position and zonation: Ecology, v. 40, p. 67-90. and N. I. Gareau, 1973, The ecology of Jamaican coral reefs ; II. Geomorphology, zonation and sedimen tary phases: Bull . Marine Sci . , v. 23, p . 399-464. James, N. P . , et a!, 1 976, Facies and __
fabric specificity of early subsea cemen tation in shallow Belize (British Hon duras) reefs : Jour. Sed. Petrology, v. 46, p. 523-544. and R. N. Ginsburg, 1979, The seaward margin of Belize Barrier and Atoll Reefs: lnternat. Assoc . Sedimen tologists Spec. Pub. 3, 1 96 p. Maxwell, W . G . H., 1 968, Atlas of the Great Barrier Reef: Amsterdam, Elsevier Sci. Pub. Co., 258 p .
__
Pinnacle Reefs Korniker, L. A . , and D. W. Boyd, 1962, Shallow water geology and environment
of Alacran reef complex, Campeche Bank, Mexico: AAPG Bull. , v. 46, p.
640-673.
Logan , B. W . , et al, 1969, Carbonate sediments and reefs , Yucatan sh elf, Mexico: AAPG Mem. 1 1 , p. 1-196.
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Garrett , P . , et al, 1 97 1 , Physiography, ecology and sediments of two Bermuda patch reefs : Jour . Geology, v. 79, p.
647-668. Ginsburg, R . N., and J. H. Schroeder, 1969, Notes for NSF seminar on car bonate cements, Bermuda Biological
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Figure 1 90-Piate-like colonies of Ac tinacis rollel are oriented pointing into and away from probable directions of wave surge in earliest Oligocene coral banks exposed in the lowe r slopes of Monti Gruml, near Castelgomberto, Venetian Alps region, Italy. The coralline marl matrix contains prolific growths of branching and nodular Goniopora {photo S. Frost).
Figure 1 9 1 -A view of the core of an Upper Eocene (Priabonian) reef mound near Barcelona, Sp ai n . These - mud-rich mounds are composed of floatstones comprising Porites(photo M . Esteban}.
Station: unpub. Maiklem , W. R . , 1970, The Capricorn Reef complex , Great Barrier Reef, Australia: Jour. Sed. Petrology, v. 38, p. 785-798.
Reef Mounds Turmel , R., and R. Swanson , 1 976, The development of Rodriguez Bank, a Holocene mudbank in the Florida Reef Tract : Jour . Sed. Petrology, v. 46, p.
497-5 19.
Algal Reefs
Adey, W., and R. Burke, 1976, Holocene bioherms (algal ridges and bank-barrier
reefs) of the eastern Caribbean : Geol. Soc. America Bull. , v. 87, p. 497-519. Ginsburg , R . N . , and J. H . Schroeder, 1 973, Growth and submarine fossiliza tion of algal cup reefs, Bermuda: Sedimentology, v. 20, p. 575-614.
Stromatolites Hoffman , P . , 1976, Stromatolite morpho genesis in Shark Bay, Western Australia, in M. R. Walter, ed. , Stromatolites: Amsterdam, Elsevier Sci. Pub . , p. 261-273. Playford, P . E . , and A. E. Cockbain, 1976, Modern algal stromatolites at
Hamelin Pool , a hypersaline barred basin in Shark Bay, Western Australia, in M. R. Walter, ed. , Stromatolites: Amsterdam, Elsevier Sci Pub . , p.
389-4 1 3 . Deepwater Mounds Mullins, H. T . , et al, 1 98 1 , Modern deep water coral mounds north of Little Bahama Bank ; criteria for recognition of deep-water coral bioherms in the rock record: Jour. Sed. Petrology, v. 5 1 , p. 999-1013. Neumann, A. C. , J . W. Kofoed, and G. H . Keller, 1 977, Lithoherms in the
434
Reef
Fig u re 1 92-A b i ohe rm rich in branching coralline algae (white shrubs) and having a matrix of lime mud from the Upper Miocene near Llucmajor, Mallorca (photo by M. Esteban).
Figure
1 93-lnset detail of Figure 1 92 .
Heckel, P. H . , 1 974, Carbonate buildups in the geologic record; a review, in L . F. Laporte, ed . , Reefs in time and space: SEPM Spec. Pub. No. 1 8, p . 90- 1 5 5 . James, N. P . , 1979, Reefs , in R. G. Walker, ed . , Facies models: Geosci. Canada Repr. Ser. 1, p. 1 2 1 - 1 3 3 . Laporte, L . F., ed . , 1 974, Reefs i n time and space: SEPM Spec. Pub. No . 1 8, 256 p. Toomey, D . F., 1 98 1 , European fossil reef models : SEPM Spec. Pub. No. 30, 545 p. Walker, K. R . , and L. P. Alberstadt, 1 975, Ecological succession as an aspect of structure in fossil communities: Paleobiol . , v. 1 , p . 238-257. Wilson, J. L . , 1975 , Carbonate facies in geologic history: Heidelberg, Springer Verlag Pub . , 471 p. Reefs at Different Times in Geologic History
Straits of Florida: Geology, v . 5,p. 4- 1 1 . Linear Mud Banks Enos, P., and R. Perkins, 1 979, Evolution of Florida Bay from island stratigraphy: Geol. Soc . America Bull . , p t . I , v . 90 , p . 59-83. Pusey , W. C., 1975, Holocene carbonate sedimentation on northern Belize Shelf, in K. F. Wantland and W. C. Pusey, eds . , Belize Shelf-carbonate sediments, clastic sediments, and ecology: AAPG Stud. in Geology No. 2, p. 1 3 1 -234.
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Copper, P . , 1974, Structure and develop ment of early Paleozoic reefs : Proc . , 2nd lnternat. Coral Reef Symp. , v. 6, p. 365-386. Dunham, R . J., 1970, Stratigraphic reefs versus ecologic reefs: AAPG Bull. , v . 54, p . 1931-1932. Hartman , W . D., J. W. Wendt, and F. Widenmayer , 1980, Living and fossil sponges: Univ. Miami, Sedimenta VIII, Comparative Sedimentology Lab . , 274 p .
Pleistocene Chappell, J . , and H . A. Polach, 1976, Holocene sea-level change and coral reef growth at Huon Peninsula, Papua New Guinea: Geol. Soc. America Bull . , v. 87, p . 235-239. James, N. P . , C. S. Stearn, and R. S. Harrison, 1977, Field guidebook to modern and Pleistocene reef car bonates, Barbados, West Indies: Univ. of Miami, 3rd InternaL Coral Reef Symp . , Fisher Island, 30 p. Mesolella, K. J . , H. A. Sealy, and R. K. Matthews, 1 970, Facies geometries within Pleistocene reefs of Barbados,
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435
Figure 1 94-The core of a sma l l bioherm rich in serpu l i d worm tubes from the Upper Miocene of Mallorca; t h e diameter of the tubes is about 1 .5 em (photo M. Esteban).
Figure 1 95-The core of a late Pleistocene barrier reef exposed along the north east coast of Barbados, West Indies. The large coral fronds and logs are a l l Acropora patmata (packsack for scale).
West Indies: AAPG Bull . , v. 54, p . 1 899- 1 9 1 7 . Stanley, S . M . , 1966, Paleoecology and diagenesis of Key Largo limestone, Florida: AAPG Bull . , v. 50, p . 1 927-1 947. Cenozoic Dabrio, C. J . , M . Esteban, and J . M . Martin, 1 98 1 , The Coral Reef Model of Nijar, Messinian (uppermost Miocene) Almeria Province, southeast Spain: Jour. Sed. Petrology, v. 5 1 , p. 521-54 1 . Esteban, M . , 1979, Significance of the Upper Miocene coral reefs of the western Mediterranean : Paleogeography, Paleoclimatology, Paleoecology, v. 29, p. 1 69- 1 89. Forman, M. J . , and S . 0. Schlanger, 1957, Tertiary reefs and associated limestone facies from Louisiana and Guam: Jour. Geology, v . 65; p . 61 1 -627. Frost, S. H . , 1977, Cenozoic reef sys tems of Caribbean-prospects for paleoecological synthesis: AAPG Stud. in Geology No . 4, p. 93- 1 10. __ 1 98 1 , Oligocene reef coral bio facies of the Vicentin , northeast Italy, in D. F. Toomey, ed . , European fossil reef models: SEPM Spec . Pub. No. 30, p. 483-54 1 . Pedley, H . M . , 1 979, Miocene bioherms and associated structures in the Upper Coralline Limestone of the Maltese Islands; their lithification and paleo en vironment: Sedimentology, v. 26, p . 577-593. Riding, R . , 1979, Origin and diagenesis of lacustrine algal bioherms at the
margin of the Reis Crater, Upper Miocene, southern Germany: Sedimen tology, v. 26, p. 645-68 1 . Cretaceous Bebout, D . G . , and R. G . Loucks, 1974, Stuart City trend, Lower Cretaceous, south Texas: Austin, Tex . , Bureau Econ. Geol . Rept. No. 78, 80 p. Enos, P., 1 974, Reefs , platforms, and basins of Middle Cretaceous in north east Mexico: AAPG Bull . , v. 58, p. 800-809. Kauffman, E. G . , and N. F. Sohl, 1 974, Structure and evolution of Antillean
Cretaceous rudist frameworks: Verhandl. Naturi. Ges. Basel . , v. 84, p. 399-467. Perkins, B. F . , 1974, Paleoecology of a rudist reef complex in the Comanche Cretaceous Glen Rose Limestone of central Texas, in B. F. Perkins , ed. , Aspects of Trinity Division Geology, Geoscience and Man VIII: La. State Univ . , p. 1 3 1 - 1 73. Philip, J . , 1972, Paleoecologie des forma tions a rudistes du Cretace Superior-l'example du sud-est de Ia France: Paleogeography,
436
Reef
Figure 1 96-A succession of three coral communities in the back-reef zone of an 83,000-year-old reef complex on the northeast coast of Barbados. The lower facies is composed of the del icate branching coral Porites porites (see hammer, lower right) overlain by Acropora pa/mata fronds (center) whic h i n turn serve as a base o f g rowth of large Montastraea annularis colonies. This sequence suggests coral growth in a leeward p rotected e nvironm en t (Porites porites), a reef c rest setting (Acropora palmata) and a reef front en vironment (Montestraea annularis).
Figure 1 97-A multi lobate colony of Pleistocene Montastraea annularis ex posed in the sawed quarry wall at Windley Key, Florida. The lens cap is 5 em in diameter.
Rutten, M. D., 1956, The Jurassic reefs of the Yonne (southeastern Paris Basin) : Amer. Jour. Sci . , v. 254, p . 363-37 1 .
Warme, J . E . , R . G. Stanley, and J . L . Wilson, 1 975, Middle Jurassic reef tract , central High Atlas , Morocco: Proc. InternaL Assoc. Sedimentol . , Theme VIII, 1 1 p.
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Paleoclimatology, Paleoecology, v. 12, p . 205-222. Scott , R. W., 1979, Depositional model of Early Cretaceous coral-algal-rudist reefs, Arizona: AAPG Bull . , v. 63 , p. 1 108- 1 128.
Jurassic Eliuk, L . S., 1979, Abenaki Formation, Nova Scotia Shelf, Canada; a deposi tional and diagenetic model for Mesozoic carbonate platforms : Cana dian Petroleum Geology Bull . , v. 24, p .
424-514. Gwinner, M. P . , 1 968, Carbonate rocks
of the Upper Jurassic in southwest Ger many, in G. Muller, ed . , Sedimentology of parts of central Europe: Heidelberg, 8th Internat. Sedimentol. Cong . , p . 1 93-207. __ 1976, Origin of the Upper Juras ic limestones of the Swabian Alb (southwest Germany): Contrib. Sedimentology, v. 5 , 75 p. Palmer, T. J., and F . T. Fursich, 1 98 1 , Ecology o f sponge reefs from the Up per Bathonian (Middle Jurassic) of Normandy: Palaeontology, v. 24, p . 1 -25 .
Bosellini, A . , and D . Rossi, 1 974, Tri assic carbonate buildups of the dolomites, northern Italy: SEPM Spec . Pub. 1 8 , p. 209-233. Flugel, E . , 1 98 1 , Paleoecology and facies of Upper Triassic reefs in the northern Calcareous Alps, in D. F. Toomey, ed . , European fossil reef models: SEPM Spec. Pub. No . 30, p. 291-361 . Leonardi, P . , 1 967, Le Dolomiti; geologic dei manti tra Isarco e Piave: Rome, Nat. Research Council, v. 1 and 2, 1010 p. Stanley, G . D . , 1979, Paleoecology, struc ture and distribution of Triassic coral buildups in western North America: Ar ticle 65 , Univ. Kansas Paleontol. Con trib . , 58 p. Zankl, H., 1 97 1 , Upper Triassic carbon ate facies in the northern Limestone Alps, in G. Muller, ed., Sedimentology of parts of central Europe: Heidelberg, Guidebook 8th Internat. Sedimentol. Cong. , p. 1 47- 1 8 5 .
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Reef 437
Figure 1 99-A si ng le large massive col· ony of Montastraea annularis in the back-reef facies of a late Pleistocene reef complex on the northeast coast of Barbados. The sediments su rrounding the coral are biotu rbated lime mudstones (hammer for scale}.
Figure 1 99-A hemispherical colony o f Dip/oria from th e late Pleistocene Key Lar g o l i mestone exposed in Windley Key Quarry, Florida. The colony has been bored by b i valves yi e ldin g e l on g ate holes. The boring next to the finger is due to boring by the date mussel Lithophaga while the others are due to the bivalve Gastrochaena.
and discussion for individual reinter pretation of the sedimentation and diagenesis of the Permian Capitan Geologic Reef and associated rocks, New Mexico and Texas : Permian Basin Sec ., SEPM Pub. 72- 14, 235 p .
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Hileman, M. E., and S. J. Mazzulo, 1977, Upper Guadal upi an Facies, Per mian Reef Complex, Guadalupe Mou tains New Mexico and Texas: Permian Basin Sec. , SEPM Pub. 77-16, 508 p. Malek-Aslani, M . , 1 970, Lower Wolfcamp reef in Kemnitz Field, Lea County, New Mexico : AAPG Bull . , v. 54, p .
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__ et al, 1976, Permian Reef Com plex, Tunisia: Brigham Young Univ. Geol. Stud . , v . 23, p. 75- 1 1 2. Smith , D. B., 1 98 1a, The Magnesian Limestone (Upper Permian) Reef Com plex of northern England, in D. F. Toomey, ed . , European fossil reef
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30, p. 161- 1 87. __ 1981b, Bryozoan-algal patch reefs in the Upper Permian Magnesian Limestone of Yorkshire, northeast
England, in D. F. Toomey, ed . , Euro pean fossil reef mo dels : SEPM Spec . Pub. No. 30, p. 187-203 . Pennsylvanian Heckel, P . H . , and J . M . Cocke, 1 969, Phylloid algal mound complexes in out cropping Upper Pennsylvanian rocks of mid-continent: AAPG Bull . , v. 53, p .
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Pray, L. C . , J. L. Wilson, and D. F. Toomey, 1 977, Geology of the Sacramento Mountains, Otero County, New Mexico: West Texas Geol. Soc. Guidebook, 216 p. Toomey, D. F., and H. D. Winland, 1973, Rock and biotic facies associated with a Middle Pennsylvanian (Des moinesian) algal buildup, Neca Lucia Field , Nolan County, Texas : AAPG Bull . , v. 57, p. 1053- 1 074.
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Reef
Figure 200-A huge, partly eroded Mon· tastraea annularis colony forming part of the Key Largo limestone in Windley Key Quarry, Florida. The colony stretches from left of the lens cap across the photograph to beyond the right-hand side. The concave upper surface was likely eroded by b rowsin g echlnolds (see Fig. 4) so that the center of the colony was dead while the periphery was still alive. The holes in the rock are diamond drill holes (lens cap is 5 em in diameter).
Figure 201 -A commu nity of branching and small domal coral colonies forming part of a late Pleistocene patch reef on the southern part of Barbados. The branches o r sticks are Acropora cer· vicornis while the two head corals in the center are Porites porites and Dip/oria. The upper part of the outcrop is oc· cupied by a cal crete (caliche} c rust, an excellent criteria for subaerial exposu re.
AAPG Bull . , v. 58, p. 787-799. Krebs, W . , 1 97 1 , Devonian reef lime
Stromatactis reefs in Lancashire, England: Jour. Geology, v. 67, p. 506-521 . Cotter, E., 1 965, Waulsortian-type car bonate banks in the Mississippian Lodgepole Formation of central Mon tana: Jour. Geology, v. 73 , p. 88 1 -888. Lees, A. , 1964, The structure and origin of the Waulsortian (Lower Car boniferous) " reefs" of west-central Eire : Phil. Trans. Royal Soc. London, Ser. B . , No. 740, p. 485-53 1 . Devonian Burchette, T. P., 1 98 1 , European Devon ian Reefs; a review of current concepts and models, in D. F .. Toomey, ed. , European fossil reef models: SEPM Spec. Pub. No . 30, p. 85-143.
Davies, G. R . , ed. , 1 975, Devonian reef complexes of Canada, I and II: Cana dian Soc. Petroleum Geols. Repr. Ser. No. I , 229 and 246 p . , respectively. Elloy, R . , 1972, Reflexions sur quelques environments recafaux du paleozoique: Bull. Centre Rech. Pau-SNPA, v. 6, p. 1 - 105. Jansa, L. F., and N. R. Fischbuch, 1 974, Evolution of a Middle and Upper Devonian sequence from a clastic coastal plain-deltaic complex into overlying carbonate reef complex and banks, Sturgeon-Mitsue area, Alberta: Geol. Survey Canada Bull. 234, 105 p. Klovan, J. E., 1974, Development of western Canadian Devonian reefs and comparison with Holocene analogues:
stones in the eastern Rhenish Shiefergebirge , in G. Muller, ed . , Sedimentology of parts of central Europe: Guidebook, Heidelberg , 8th Intemat. Sedimentol . Cong. , p. 45-8 1 . __ and E . W . Mountjoy, 1972, Com parison of central European and western Canadian Devonian reef com plexes: 24th Internat. Geol. Cong. , sec . 6, p. 294-309. Mountjoy, E. W., and R. Riding, 1 98 1 , Foreslope Renalcis-stromatoporoid bioherm with evidence of early cemen tation, Devonian Ancient Wall reef complex: Sedimentology, v. 28, p. 299-32 1 . Playford, P . E . , 1980, Devonian " Great Barrier Reef ' of the Canning Basin, Western Australia: AAPG Bull . , v. 64, p . 8 1 4-840. __ and D. C. Lowry, 1 966, Devonian reef complexes of the Canning Basin, Western Australia: Geol. Survey Western Australia, Bull. 1 1 8 , 50 p. Silurian Bourque, P. A . , 1 979, Facies of the Silur ian West Point reef complex, Baie des Chaleurs, Gaspesie, Quebec: Geol . Assoc . Canada Field Guidebook B-2, 29 p. Fisher, J. H . , eel., 1977, Reefs and evaporites-concepts and depositional models: AAPG Stud. in Geology No . 5, 196 p . Heckel, P . H . , and D. O'Brien, eds. , 1975, Silurian reefs of Great Lakes Region of North America: AAPG Repr. Ser. No. 14, 243 p.
Reef
Figure 202-An idealized isometric diagam i l l ustrating the facies on a car bonate she l f o r platform at times in geolog ic history when a full spectrum of reef-bu ilding skeletal metazoans are pre sent (see Figure 6 1 ). The platform margin is a zoned barrier reef and isolated patch reefs dot the shallow plat· form behind. Reef mounds are found on the inner part of the platform and on the fore- reef slope.
man, 1 975, Lithology and origin of Middle Ordovician Calcareous mud mound at Meiklejohn Peak, southern Nevada: U.S. Geol. Survey Prof. Paper No. 87 1 , 45 p. Toomey, D . F. , 1970, An unhurried look at a Lower Ordivician mound horizon, southern Franklin Mountains, west Texas : Jour. Sed. Petrology, v. 40, p. 1 3 1 8- 1 335.
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v. 58, p. 430-487. Manten, A. A . , 1 97 1 , Silurian reefs of Gotland, in Developments in Sedimen tology, No. 1 3 : Amsterdam, Elsevier Sci. Pub . , 539 p . Riding, R., 1 98 1 , Composition, structure and environmental setting of Silurian bioherms and biostromes in northern Europe, in D. F. Toomey, ed . , Euro pean fossil reef models: SEPM Spec. Pub. No. 30, p. 41 -85 . Scoffin, T. P . , 1 97 1 , The conditions of growth of the Wenlock reefs of Shrop shire, England: Sedimentology, v. 1 7 , p . 173-2 1 9 . Shaver, R . H . , et al, 1978, The search for a Silurian reef model; Great Lakes Area: Spec. Rept. No . 1 5 , Indiana Geol. Survey, 36 p .
Ordovician Klappa, C. F. , and N. P . James, 1980, Small Lithistid sponge bioherms, Early Middle Ordovician Table Head Group, western Newfoundland : Bull. Canadian Petroleum Geology, v. 28, p. 425-45 1 . Pitcher, M . , 1 961 , Evolution o f Chazyan (Ordovician) reefs of eastern United States and Canada: Canadian Petroleum Geology Bull . , v. 1 2 , p . 632-691 .
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and R. M . Finks, 1 969, The paleo
ecology of Chazyan (lower Middle Or
dovician) "reefs" or "mounds" and Middle Ordovician (Chazyan) mounds, southern Quebec, Canada: Plattsburg, N . Y . , New York Assoc . Guidebook to Field Excursions, College Arts, Scies. (a summary rept. ) , p. 1 2 1 - 134. __ and M. H. Nitecki, 1 979, Organic buildups in the Lower Ordovician (Canadian) of Texas and Oklahoma:
Fieldiana N. Ser. 2, 1 8 1 p . Walker, K . R . , and K. F. Ferrigno, 1 973, Major Middle Ordovician reef tract in eastern Tennessee: Am. Jour. Sci. , v . 273-A, p. 294-325 .
Cambrian Ahr, W . M . , 197 1 , Paleoenvironment, algal structures and fossil algae in the Upper Cambrian of central Texas: Jour. Sed. Petrology, v. 4 1 , p. 205-21 6. James, N. P . , and D. R. Kobluk, 1978, Lower Cambrian patch reefs and associated sediments, southern Labrador, Canada: Sedimentology, v . 25, p . 1-32. __ and F. Debrenne, 1 980, Lower Cambrian bioherms; pioneer reefs of the Phanerozoic: Acta. Palaeontologica Polonica, v. 25, p. 655-668.
Precambrian Cecile, M . P . , and F. H. A. Campbell, 1 978, Regressive stromatolite reefs and
439
associated facies, middle Goulburn
Group (Lower Proterozoic) in Kilohigok Basin, N.W.T.; an example of environmental control of stromatolite form: Bull. Canadian Petroleum Geols . , v. 26, p. 237-267. Hoffman, P . , 1974, Shallow and deep water stromatolites in lower Proterozoic platform-to-basin facies change, Great Slave Lake, Canada: AAPG Bull . , v. 58, p. 856-867 . Semikhatov, M. A . , et al, 1979, Stroma tolite morphogenesis-progress and prob lems: Canadian Jour. Earth Sci . , v. 1 6, p. 992- 1 01 6 . Other References Cited
Aitken, J . D . , 1967, Classification and environmental significance of cryptalgal limestones and dolomites with illustra tions from the Cambrian and Ordovi cian of southwestern Alberta: Jour. Sed. Petrology, v. 37, p. 1 1 63- 1 1 87 . Frost , S . H . , 1977, Ecologic controls of Caribbean and Mediterranean Oligocene reef coral communities, in D . L . Taylor, ed . , Miami, Fla. , Proc. 3rd Internat. Coral Reef Symp. , p. 367-375 . Ginsburg, R. N . , and N . P. James, 1 974, Holocene carbonate sediments of con tinental shelves, in C. A. Burk and C. L . Drake, eds., The geology of con· tinental margins : New York, Springer Verlag Pub . , p. 1 37- 1 57 . __ J . H . Schroeder, and E . A . Shinn, 197 1 , Recent synsedimentary cementa tion in subtidal Bermuda reefs, in 0. P. Bricker, ed . , Carbonate cements : The Johns Hopkins Press, p . 54-56. lams, W. J . , 1 970, Boilers on Bermuda's South Shore, in R. N. Ginsburg and S . M . Stanley, eds . , Reports o f research, 1969, seminar on organism-sediment in-
440
Reef
Figure 203-An idealized isometric diagram i l l ustrating the facies on a car bonate shelf o r platform at times in geologic history when the only skeletal invertebrates with reef-bu ilding potential are delicate ramose or encr usti ng forms and so reef mounds are the only types of bui ldups. The platform margin is a series of carbonate sand shoals and the reef mounds are restricted to the tran quil inner parts of the platform and down slope in deeper water in front of the platform.
CYC L E I I CYC L E I
Figure 204-A generalized plot of the main biotic constituents of ca rbonate buildups against geologic time for the Phanerozoic. The ba l loons represent the relative abundance and importance of the different taxa. The plot is sepa rated into two apparent cycles, each of which illustrates a similar evolution in reef· building taxa with time.
terrelationships: Bda. Bio. Stn. Spec. Pub. No. 6, p. 9 1 -99. Shinn, E. A . , et al, 1 977, Topographic control and accumulation rate of some Holocene coral reefs; south florida and Dry Tortugas , in D. L. Taylor, ed . ,
Proceedings of third international coral reef symposium, Miami: Geology, p . 1-9. Wallace, J . , and S. D. Schafersman, 1977, Patch-reef ecology and sedimen tology of Glovers Reef Atoll, Belize, in
S. H . Frost, M . P. Weiss, and J . B. Sanders, eds., Reefs and related car bonates-ecology and sedimentology: AAPG Stud. in Geology No. 4, p . 37-53.
Reef
441
Lower Cretaceous Reefs, South Texas Don G. Bebout Robert G . Loucks
! I
L
ower Cretaceous shelf car Aptian, Albian, and Cenomanian age ac cumulated in a broad band which completely circled the Gulf of bonates of
Mexico (Figs. I , 2). These carbonates
are well
known
on the outcrop in
eastern Mexico and central Texas and
i n the subsurface of the Yucatan Peninsula in eastern Mexico, south
and east Texas, central and south Louisiana, south Florida, and th e Bahamas. Throughout this Lower Cretaceous shelf area, water depths ranged from a few feet to one or two hundred feet and warm-water marine organisms flourished . Along a narrow band on the basinward edge of this broad shelf, biogenic growth climaxed and a complex of reefs, banks, bars, and islands developed. These Lower Cretaceous shelf-margin carbonates attained a thickness of 2,000 to 2 ,500 ft and have been identified as an almost continuous belt around the en tire Gulf of Mexico. The limestone making up this shelf margin complex, the Stuart City For mation, is composed of reef and bank carbonates constructed primarily of rudist pelecypods and various types of carbonate sand bodies. Rudists are a group of sessile pelecypods which attached to each other or hard substrate to form boundstones and framestones. Other organisms which aided the rudist in forming bound stones and framestones of the shelf edge are encrusting algae, encrusting bryozoans, stromatoporoids, and corals. Five major environmental units are recognized : open marine, lower shelf slope, upper shelf slope, shelf margin,
Shelf carbonates � Shelf IY margin trend
Terrigenous D clastics 0
o
I 00
20'J
100
-
�00
kilL
200mi. I
Figure 1-Distribution of Lower Cretaceous shelf-marg i n facies around the Gulf of Mexico (after Meyerhoff,
1 967).
and shelf lagoon (Fig. 3). The sea floor in the open-marine environment was probably under more than 60 ft of water, and reducing conditions ex isted within the sediment . The amount of influx of siliceous material varied greatly from time to time and from place to place. Tests of planktonic foraminifers rained down to accumulate on the bottom along with very small plates and spines o f
echinoids and small mollusks, animals which lived there. Landward, the sea shallowed very gradually, resulting in the slightly shallower conditions on the shelf slope. On the lower shelf slope, echinoids and mollusks became more abundant and the individuals larger . Water depths probably ranged beCopyright
©
1983 by The American Association ol Petroleum Geologists.
442
Reef
f I
OKLAHOMA
Figure 2-Paleogeography of the lower Cretaceous of Texas, (after Fisher and Rodda, 1969).
Figure 3-Facies and interpreted depositional environments across the Stuart City Trend South Texas (from Bebout and Loucks, 1 974) ,
.
MEXICO U SO �
MILES
Reef
Fig u re 5-Requienid boundstone (ba r scale 1 em). =
Figure 6-Rudist grainstone (bar scale =
tween 30 and 60 ft ; local submarine currents concentrated these skeletal elements into channels although most of the bottom was mud. The tenden cy toward shallower water and more oxidizing conditions continued land ward onto the upper shelf slope where water depths ranged between 10 and 50 ft . The bottom continued to be predominantly mud, but a larger number and greater variety of animals were able to live in this area. The most significant fauna added to those of the lower shelf slope are caprinid rudists and branching and massive corals. Locally, patch reefs or banks composed of corals and stromatoporoids grew. These patch reefs were isolated from the shelf margin banks and reefs by wide areas of mud bottom with caprinids and corals . The shelf-margin facies include coral-caprinid boundstone, requienid Figure 7-Caprinid-coral wackestone 1 em). (bar scale =
443
1 em).
boundstone (Figs. 4, 5), rudist grainstone (Fig . 6), algae-encrusted miliolid-coral-caprinid packstone, and caprinid-coral wackestone (Fig . 7). The shelf edge is characterized as having been composed of a series of requienid banks and patch reefs with associated rudist sand bodies. Struc tures within the sand units indicate that a complex of environments is represented, including bars, tidal channels, islands, and beaches. On the seaward side of this band, coral caprinid banks and reefs occurred more sporadically. None of the shelf edge units formed a continuous band; instead, they were elongate, narrow bodies aligned parallel to the shelf edge, except for the channels which were perpendicular to the edge. Be tween the banks, reefs, and bars, caprinid rudists and branching corals Jived on a mud bottom, thus resulting in the accumulation of the caprinid-
444
Reef
coral wackestone facies . In some areas, a thick sequence of algae coated grains accumulated on a broad flat covered by very shallow water . The shallow-water shelf-lagoon facies include miliolid wackestone, mollusk wackestone, toucasid wackestone, and mollusk-miliolid grainstone. The facies accumulated in shallow water under low-energy con ditions. Local variations in currents or bottom topography resulted in the formation of small islands typified by well-developed bird' s-eye structures; these islands were commonly fringed by mollusk grainstone bodies. Grapestone and crusts developed on broad, very shallow-water stable flats in areas protected from normal surf and tidal currents. Small channels served as passageways for the slow movement of sea water on and off the flats during tidal changes but significant sediment transport took place on these flats only during major storms . Cementation between grains, within voids in boundstone areas , and within
body cavities of rudist and other mollusks occurred at several places in this environmental setting, but primarily within the facies of the shelf lagoon and shelf margin. Micrite rims, the result of both grain edge alteration and algal coating, are present on all the skeletal fragments and intraclasts in the grainstone facies. These rims formed as the grains lay on the sea floor . Thin layers of marine fibrous to bladed isopachous cement were deposited within voids and between grains in facies which were accumulating in the very shallow, warm water. Dripstone and meniscus cements were deposited within the sediments exposed to vadose or intertidal conditions on small, local islands. The micrite rims, isopachous cement , and dripstone and meniscus cement, however , make up a very small part of the final calcite cementation and merely serve to par tially lithify the sediment and protect it from later compaction. After additional growth of the shelf margin, accompanied by slow sub-
sidence of the shelf, these partially cemented sediments were buried to shallow depths. Leaching of aragonite and high-magnesium calcite shells took place. All voids were lined with thick layers of impure radiaxial ce ment deposited from phreatic meteoric water . Later, after addi tional subsidence, the remaining voids were filled with equant calcite .
SELECTED REFERENCES Bebout , D. G . , and R. G. Loucks, 1 974, Stuart City Trend, Lower Cretaceous, South Texas-a carbonate shelf-margin model for hydrocarbon exploration: Austin , Univ . Texas , Bur. Econ . Geology Rept. Inv. 78, 80 p . Fisher, W . L . , and P . U . Rodda, 1 969, Edwards Formation (Lower Cretaceous), Texas-dolomitization in a carbonate platform system : AAPG Bull . , v. 53, p. 55-72. Meyerhoff, A. A., 1967, Future hydrocar bon provinces of Gulf of Mexico Caribbean region: Trans., Gulf Coast Assoc. Geol. Socs . , v. 1 7, p. 2 1 7-260.
Reef
445
Golden Spike Reef Complex, Alberta R i chard A. Wal ls
i i
i
U
pper Devonian (Frasnian) carbonate buildups in Alberta have been the sub ject of numerous studies since oil was discovered in them over 30 years ago . Of particular impor tance are Leduc carbonates which are known to contain 4.5 billion barrels of oil and 1 7 trillion cubic feet of gas in place. Leduc carbonates comprise a series of platformal (biostromal) , reef and biohermal, and skeletal bank deposits that occur in the subsurface of the Western Canada basin and in various thrust sheets of the Rocky Mountains. This paper discusses the deposi tional and diagenetic history as well as the reservoir properties of Golden
r
!
lAL B E R T A l
�: \"' \
i'·
I
, \
·....
..
i
-·-.J
Figure 1 -Location of the Golden Spike reef complex in the Alberta Basin, with structure contours on the Leduc
N i s ku Fm .
Formation.
Figure 2-Subsu rface and surface stratigraphic terminology for the Fras nian (Upper Devonian) In Alberta. Reef complexes are colored blue .
rways
Copyright@ 1983 by The American Association ol Petrolel.fl'l Geologists.
446
Reef
TABLE 1
Environment
Zones
Major Facies
Subfacles
General
Reef Margin
Reef flank
*Detrital coral
Seaward of active reef growth; shallow-water low-relief fore-reef slope Fore-reef, similar to detrital coral but more in situ growth forms
Tabular stromatoporoid ArgiUaceous, skeletal rubble Massive ·stromatoporoid
Reefs Reef Interior
Subtidal skeletal sand flats
Stromatopo«Md r�e Skeletal sand Skeletal,
Peritidal algal laminites
Patch reefs
peloidal• sand
"Deeper" tore-reef with mixture of fore-reef detritus and basin mud
Shallow-water, well-washed zone of reef growth Accumulations of "eroded� reef framework Shallow-water, baCk-reef flat; "apron• of skeletal detritus Well-washed, subtidal, back-reef shoals lagoons
and
Intertidal to supratidal algal mud flats Localized reef growth in subtidal, back-reef flats and lagoons *Detrital refers to reef-building skeletal components which llle not in situ.
Table 1 . Summary of the major facies, subfacies, and general environments of deposition for Golden Spike.
Figure 3-East-to-west cross section through Golden Spike ill ustrating the ma· jor facies. Datum is the top of the Beaverhill Lake Group.
Spike, one of these Leduc buildups . The Golden Spike reef complex (limestone) is located 34 km southwest of Edmonton, and just 4 km west of the dolomitized Leduc Rimbey reef trend of central Alberta ( Fig. 1 ) . It is one of the smallest and most prolific oil producing (2 1 3 million barrels , recoverable reserves) , Upper Devonian carbonate complexes in Western Canada, being only 10 sq km in area of closure but ranging up
to 1 97 m thick in the interior and thinning to less than 30 m on the flanks . It was discovered in 1 949 by Imperial Oil Ltd . STRATIGRAPHY: MAJOR FACIES
Rocks of the Golden Spike reef complex are part of the Upper Devo nian Leduc Formation (Fig. 2) ami have been subdivided by McGillivray and Mountjoy ( 1 975) and McGillivray
( 1 977) i n to a lower Leduc platform , or biostromal unit , a middle Leduc bioherm unit, and an upper Leduc reef-fringed bank. The lower Leduc platform sequence is comprised of nonreservoir rocks that c o v er a 3 1 sq km area and are correlative with the Cooking Lake Formation to the east . The main reef complex (middle and upper Leduc) is considerably smaller ( 1 0 sq km), and is subdivided mor phologically into a reef margin and
Reef
Figure 4-Core s l ab of a tabu lar stromatoporoid l i me packstone contain ing a submarine fracture that is f i ll ed with a n ostracod, peloidal grai nstone in ternal sediment (i). The fracture trun cates a banded radiaxial calcite cement (R). Well 1 2-24, 5829 ft.
reef interior (Table 1 , Fig. 3) . Basin or off-reef sediments are referred to as the Duvernay and Ireton forma tions (Fig . 2). Reef Margin Golden Spike's reef margin (Figs. 4 to 8) is characterized by a reef flank (fore-reef) and reef facies. Reef Flank Rocks of the reef flank are com posed of a mixture of poorly sorted skeletal debris, minor in situ stromatoporoids, encrusting stromatoporoids and algae, and abun dant fine grained carbonate sediments (fine skeletal grainstones , lime mudstones , and calcareous shales) . Banded radiaxial calcite is a common "binding" agent (cement) for the sediments of the reef flank. A tabular stromatoporoid facies (Fig . 4}, observed in most wells of the reef margin, is characterized by both detrital and in situ tabular stromatoporoids. Renalcids (prob lematic algae or foraminifera) often
Fi gu re 5-Core slab of a tabular st ro m ato poroid l i me packstone with
several generations of marine internal sediments (i) filling large cavity sys tems Bar scale is in millimeters. Well 9-27, 5558 ft.
appear as encrustations on stromatoporoids and as discrete grains. Other fossil constituents in clude abundant crinoid plates and less common corals, brachiopods, pelecypods, and gastropods generally set in poorly sorted lime packstones and wackestones . Marine geopetal sediments (Fig. 5) are also common in this facies , generally occurring as complete to partial fillings in small (5 mm to 5 em) shelter and growth framework voids. The tabular stromatoporoid facies occurs basin ward of the reef in shallow water of a low relief, fore-reef slope. A detrital coral facies (Fig. 6) is observed locally on the reef margin and is less common than the tabular stromatoporoid facies . It is composed of corals ( Thamnopora, Coenites, Phil/ipsastrea, Alveolites) as well as stromatoporoids, crinoids, renalcids , and rare brachiopods, pelecypods, gastropods and ostracods . The detrital coral facies represents local ized fore-reef deposition laterally equivalent to the tabular stroma-
.
447
Figure 6-Core slab of a tabulate coral lime packstone f rom the det rit al coral facies. Radiaxial calcite cement (R) is common in this facies, and porosity (ar rows) generally is low. Well 1 2·26, 5733 ft.
toporoid facies. An argillaceous skeletal rubble facies contains massive and tabular stromatoporoids, rugose and tabulate corals, and crinoid fragments in an argillaceous lime packstone matrix . Green to tan calcareous shales that occur throughout this facies as matrix, geopetal sediments, and as thin partings were probably deposited as a suspended load from the nearby basinal calcareous shales. Reef Rocks of the reef facies are com posed of massive and tabular stromatoporoids occurring in poorly sorted lime grainstones (rudstones) or packstones (Fig. 7). A massive stromatoporoid boundstone and stromatoporoid rubble facies com prise the reef zone. Boundstones are found locally in all wells of the reef margin, but are probably best developed in wells on the northwest margin . They are characterized by an abundance of massive, and less com monly tabular stromatoporoids. Well
448
Reef
:T RIC
1
2
Figure 8-Core slab of a stromatoporoid
rubble that is cemented by banded .radiaxial calcite (R). Well 9-27, 5798 ft.
'
.,
Figure 7-Core slab of bul bous and
massive stromatoporoids from the reef facies. Well 1 2-24, 591 8 ft .
developed growth framework cavity systems are present in some cases, commonly filled with internal sediments and/or cement. Boundstones are intercalated with coarse skeletal grainstones (stroma toporoid rubble facies) composed of poorly sorted massive and branching stromatoporoids, corals, and crinoids (Fig. 8) . Reef Interior In Golden Spike, the reef interior (Figs. 9 to 1 1 ) is designated as a zone of subtidal to supratidal sedimenta tion that lies behind and is, in part, surrounded by an accretionary reef margin. Major facies of the reef in terior include subtidal skeletal sand flats, peritidal algal laminites, and minor stromatoporoid patch reefs .
The subtidal skeletal sand flats are thick accumulations of both coarse and fine grained skeletal , peloidal grainstones and packstones (Fig. 9). Skeletal components include strongly abraded stromatoporoids and corals as well as crinoids, brachiopods, gastropods , pelecypods , ostracods , and calcispheres . Coarse grained skeletal grainstones are often in terbedded with fine grained, well sorted peloidal grainstones (Fig. 1 0) . These deposits represent deposition in the shallow subtidal portions of well washed reef flats or shoals with skeletal material derived from the reef margin. Peritidal algal laminites are most common in the upper Leduc (Fig. 1 1 ) where they are characterized by light gray to tan laminated, peloidal lime mudstones and fine grained peloidal lime grainstone (Fig. 1 1) . Algal lamination is discontinuous and un dulatory micritic bands that parallel bedding contain "trapped" or bound peloidal and skeletal grains . Lami noid, tubular, and irregular fenestrae (bird's-eyes) occur throughout this facies. These laminites represent high subtidal to supratidal semi-restricted deposition. Several laterally discon-
tinuous paleoexposure surfaces h ave been documented in this facies . Patch reefs are identified by the isolated occurrence of massive and branching (Stachyodes) stromatoporoids within the skeletal sand flat facies. Patch reefs range up to 1 0 m in thickness and represent localized biohermal growth within back reef areas of skeletal sand deposition. DEPOSITIONAL HISTORY
The middle and upper Leduc at Golden Spike developed as a small reef complex isolated in a shale basin. Its history is similar in many aspects to other Upper Devonian reefs in Alberta (Andrichuck, 1 958; Klovan, 1 964, 1 974; Mountjoy, 1 967 ; Noble, 1 970; Mountjoy and MacKenzie, 1 973). The distribution of the aforementioned facies through Leduc time indicates (Fig . 1 2) : ( 1 ) shallower water conditions occurring over the "muddy" lower Leduc platform (reef foundation); (2) widespread reef facies at the base of the middle Leduc; (3) extensive shallow water deposition of reef interior facies throughout the middle Leduc with
Reef
449
Fig u re 9-Core slab of a skeletal lime gra inston e from the reef interior that contains moldic and i nterparticle porosi· ty. Well 1 0-27, 5560 ft.
Figure 1 0-Core slab of a bu rrow mot tled peloidal lime grainstone from the reef interior. Wel l 1 1 -23, 5695 ft.
seaward accretion of the subtidal skeletal sand flats resulting in only localized reef development on the margins of the complex by upper Leduc time; (4) relatively continuous sedimentation on the margins with possible periodic exposure in the in terior (peritidal laminite facies) throughout the upper Leduc; and (5) termination of reef development and eventual basin filling and burial by Ireton shales. RESERVOIR QUALITY
Reservoir development in Golden Spike involved the combination of several sedimentological and diagenetic processes that influenced (enlarged or reduced) primary porosi ty. In the reef margin, shelter, growth framework , interparticle, and in traparticle pores are often filled by a combination of cements and internal sediments which leads to an occlusion of primary porosity (Table 2) . On the other hand, skeletal , peloidal lime grainstones and laminated fenestral mudstones of the reef interior often show an enlargement of interparticle and fenestral pores by solution as well as some reduction by carbonate cements (Table 2) . Porosity distribution shows several trends throughout the reef complex
(Table 2) : (1) Porosity in the margin, par ticularly in reef flank facies is, with few exceptions, consistently low. Tabular stromatoporoids and detrital coral facies illustrate some of the lowest porosity in the reef due to pore occlusion by cementation and deposition of marine internal sediments . Argillaceous skeletal rub ble facies generally contain low
primary porosity due to a matrix (mud) supported fabric. (2) Reef facies illustrate both solu tion enlarged and cement reduced primary pores, as well as unaltered primary porosity. (3) Porosity in the reef interior is variable. Subtidal skeletal sand flat deposits contain some of the highest porosity in the reef complex, due to preservation of original interparticle
450
Reef
TABLE 2
Porosity (.p) Mean Mode
c
-;. � Ill
Permeability (K)
Mean
Mode
(md)
(md)
Major Diagenetic: Effects on q,, Kh .p, Kh
Increase
Detrital Coral
4
2
1
1
Rare Solution
Extensive Radiaxial & Fibrous Calcite Cementation.
Tabular Stromatoporoid
5.5
4
10
5
Rare Solution
Radiaxial & Abrous Calcite Cementation Deposition Of Internal Sediment.
-
I
�
0
-.::: s .5 -
I
a:
Decrease
(o/o)
:I a:
.P, Kh
(%)
Massive Stromatoporoid & Stromatoporoid Rubble
10
8.5
1 00
1 0; 1 00
Skeletal, Peloidal Sands
1 2. 5
10
50
20
Algal Laminites
7
6
60
10
Moldic Solution
Clear, Non-Ferroan & Ferman Calcite Cements.
Moldic Solution
Fibrous, Non-Ferroan & Ferroan Calcite Cements.
Sol ution Enlargement Of Fenestral Pores
Microstalacthic Cement, Non Ferroan & Ferroan Calcite Cementation
Table 2 . Reservoir properties and major diagenetic influences on porosity and permeability for the different facies in Golden Spike .
and fenestral porosity, solution of early cements, and solution enlarge ment of primary pores . DIAGENETIC CONTROLS ON RESERVOIR QUALITY
The diagenetic history of Golden Spike is complex but may be sub divided into at least four general stages (Fig. 1 3): ( l ) submarine and early burial (marine); (2) subaerial (during exposure periods, particularly in the upper Leduc); (3) middle or in termediate burial ; and (4) late or deeper burial . These stages are interpreted on the basis of certain diagnostic textural characteristics of different cements, their distribution, superposition , and relationships with associated sediments. For detailed discussions on the diagenetic history of Golden Spike see Walls ( 1 977) and Walls,
Mountjoy and Fritz ( 1 979) . The distribution of porosity in Golden Spike is affected by varying degrees of cementation and solution that occur during the four general diagenetic stages . The relative impor tance of these processes in reducing or increasing depositional porosity in the reef interior and reef margin are as follows: ( 1 ) Reef Interior : (a) minor precipitation of marine cements (fibrous calcite cements , and early burial, non-ferron clear calcite cements); (b) partial solution of these early cements and associated sediments during exposure producing vug and moldic porosity; (c) vadose and/or phreatic cementation by fibrous, micro-stalactitic and non ferroan calcites, followed by further subaerial solution, and cementation by clear calcites during middle burial, and ferroan calcites during late
burial; and (d) minor late solution associated with stylolite formation and fracturing. (2) Reef Margin : (a) precipitation of submarine fibrous cements, now represented by radiaxial calcite, in large interconnected shelter, growth framework, and "stromatactis" type cavity systems; (b) precipitation o f submarine and early burial (marine) non-ferroan syntaxial overgrowths (on crinoids) and minor clear calcites ; (c) minor early solution (subaerial) which affects the reef facies but generally does not affect reef flank facies; (d) cementation by non ferroan and minor ferroan calcites during middle and/ or late burial diagenetic stages; and (e) minor late solution associated with stylolite for mation and fracturing. Submarine and subaerial cementa tion stages are important in early (precompaction) occlusion of primary
Reef
451
Figure 1 1 -Core slab of an algal laminated l ime mudstone with fenestral porosity. Well 1 1 -23, 5556 ft.
'
\
\
i
\
I
I
I
I \
\
\
\ \ \ \
\
\
\�"'-. "
-......
_
___
TOP OF M I DDLE LEDUC
&ASE OF M l DDLE LEDUC
lEGEND R£ff INTo�IOll
n t��1��{A��faTJ\�
�ff MA-,GIN
�
UEfS
eAS!NAl SHAlt
........ ... _._ _ _,._ . ....-
NEAR TOP OF UPPER LEDUC
PL AN V I E W OF - LEDUC fAC I ES -
Figure 1 2-Pian view of the depositional history of the Golden Spike reef com· plex.
452
Reef
DIAGENESIS
-
Figure 1 3-Schematlc summary of the different stages of cementation and solu· tion in Go lden Spike.
porosity. However, high porosity zones in the reef interior are predominantly the result of solution which partially or completely remov ed some of the early cements as well as enlarged other primary pores. Zones of porous rocks intercalated with zones of cemented rocks occur throughout the reef interior, and these cemented zones form vertical permeability barriers. Permeability barriers of a cementation origin are obstacles to hydrocarbon recovery; therefore a complete knowledge of the diagenetic history of carbonate reservoirs is necessary for efficient reservoir management. SUMMARY
Golden Spike is an Upper Devo nian limestone reservoir that developed as a small reef complex in a shallow open marine basin only 4 km from a major linear reef barrier (Leduc-Rimbey reef trend). Although only 10 sq km in area of closure, it contains 2 1 3 million barrels of recoverable oil. The major facies comprising Golden Spike are the reef margin and
-
-
reef interior. The reef margin is com posed of: (a) the reef flank-tabular stromatoporoid, detrital coral, and argillaceous skeletal rubble; and (b) the reef-massive stromatoporoid boundstone, and stromatoporoid rub ble. The reef interior consists of sub tidal skeletal sand flats, peritidal algal laminites, and patch reefs. Porosity in reef margin facies, although extremely variable, is con sistently lower than in the reef in teriro . Large shelter and growth framework pores in the reef flank are commonly filled with "early" cements and geopetal sediments . On the other hand, subtidal skeletal sand flat deposits and peritidal laminites of the reef interior contain some of the highest porosity due to preservation of original interparticle and fenestral porosity, solution enlargement of primary pores, and solution of early cements. The distribution of porosity in Golden Spike is affected by varying degrees of cementation and solution that occurred during four general diagenetic stages: (1) submarine and early burial (marine); (2) subaerial; (3) middle or intermediate burial; and
(4) late or deeper burial. Submarine and early burial cemen tation reduced porosity considerably in the reef margin. In the reef in terior, periodic exposure and accom panying subaerial diagenesis produced alternating zones of tight (cemented), and porous rocks.
REFERENCES CITED Andrichuck, J. M . , 1958, Stratigraphy and facies analysis of Upper Devonian reefs in Leduc, Stettler , and Redwater areas, Alberta: AAPG Bull . , v. 24, p . 1-93. .Klovan, J. E., 1 964, Facies analysis of the Redwater reef complex, Alberta, Canada: Canadian Soc. Petroleum Geols. Bull. , v. 12, p. 1 -100. __ 1 974, Development of western Canadian Devonian reefs and com parison with Holocene analogues : AAPG Bull. , v. 58, p. 787-799. McGillivray, J. G . , 1977, Golden Spike D3A poo l , in I. A. Mcllreath and R. D. Harrison, eds. , The geology of selected carbonate oil, gas, and lead zinc reservoirs in western Canada: Canadian Soc. Petroleum Geols. , p. 67-88. __ and E. W. Mo untj o y , 1975, Fa-
Reef
cies and related reservoir characteristics, Golden Spike reef complex, Alberta: Canadian Soc. Petroleum Geols. Bull . , v. 23, p . 753-809. Mountjoy, E. W. , 1967, Factors govern ing the development of Frasnian Miette and Ancient Wall reef complexes (banks and biostromes), Alberta, in D. H. Oswald, ed . , International sym posium on the Devonian system : Calgary, Alberta Soc. Petroleum Geols. , v. 2, p. 387-408.
and W. S. MacKenzie, 1973, Stratigraphy of the southern part of the Devonian Ancient Wall carbonate com plex, Jasper National Park, Alberta: Geol. Survey Canada, paper 72-70, 1 2 1 p. Noble, J. P. A . , 1970, Biofacies analyses , Cairn Formation of Miette reef com plex (Upper Devonian) , Jasper National Park, Alberta: Canadian Soc. Petroleum Geols. Bull . , v. 1 8, p. 493-543.
__
453
Walls, R. A . , 1977, Cementation history and porosity development, Golden Spike reef complex (Devonian), Alber ta: Montreal, McGill Univ . , Ph .D. thesis, 307 p. __ E. W. Mountjoy, and P. Fritz, 1979, Isotopic composition and diagenetic history of carbonate cements in Devonian Golden Spike reef, Alber ta, Canada: Geol. Soc. America Bull . , v. 90, p. 963-982.
454
Reef
Platy Algal Reef Mounds, Paradox Basin Philip W. Choquette
R
eef mounds in which platy codiacean algae were im portant in the construction of carbonate-sediment build-ups are common in Late Paleozoic (Middle Pennsylvanian to Wolfcampian) shelf carbonates of the southwestern United States. Accord ing to Wilson ( 1 975), they generally occur along shelf margins of that region , rimming oil and gas produc ing basins in the subsurface of New Mexico and Utah, Texas and Oklahoma; they are exposed in out crops at basin margins in areas of Colorado (Eagle basin) and New Mexico (Orogrande basin). In the Paradox basin, southwestern United States (Fig. 1 ) , reef-mound buildups of platy-algal carbonate 0 50 KM packstone and grainstone have re 0 30 M l ceived particular attention because of their importance as oil and gas reser voir rocks in some two dozen fields. The largest of these, Aneth field, had Figure 1 -Genera l ized map of Middle produced about 292 million barrels of Penn sylvanian (Desmoinesian) facies in oil and gas condensate and 28 1 billion the Paradox bas i n , showing a reas of platy-a lgal reef mounds. Modified f rom cubic feet of gas by the end of 1 978 Wilson (1 975) and Choquette and Traut (Petroleum Data Systems, Dept. of ( 1 963). Energy, 1 98 1 ) . Other fields such as Desert Creek , Barker Dome, Cahone Mesa, White Mesa, Ratherford, and Ismay are smaller with cumulative productions on the order of 1 to 1 0 GENERAL SETTING AND million barrels of oil and 1 billion cu OCCURRENCE ft of gas. The economic importance specifically of the reef-mound reser The reef mounds occur along the voirs in these fields is difficult to southwest margin of the Paradox estimate because in most cases other carbonate facies are producing as well basin, in an area about 1 80 km in and production is comingled . It seems length by as much as 50 or 60 km in width (Fig . 1 ) . The Paradox basin is probable, however, that production a broad, asymmetrical sag of Late of oil and gas condensate from reef Paleozoic age. In Desmoinesian time, mound reservoirs in the Paradox the central part of the basin received basin to date is in the hundreds of upwards of 2000 m of evaporites millions of barrels.
AREA OF -..._ � ALGAL •
MOUNDS
OIL OR GAS F I ELD
(both salt and anhydrite) with rhythmically interstratified black shales; this sequence is the Paradox Formation . The broad tract of reef mound and associated carbonates and cyclically repeated thin dark shales equivalent to this sequence along the southwest margin of the basin is about 300 to 700 m thick and is known there as the Hermosa Forma tion. Siliciclastic sediments were shed into the basin from at least one bordering land mass, the Uncom pahgre uplift (Fig . 1), but in the algal reef-mound belt along the opposing margin of the basin, siliciclastic Copyright©
1983 by The
American Association of Petroleum Geologists.
Reef
CYCLES
LITHOLOGY
455
STRAT. UN ITS
Anhydrite. massive anhy. brown, silty Alternating light-<;alared lime mudstone & dark shaly m""•'"""' D<>l<>mite.
eo
50
200
platy-alga/ l i mestone 1 reef
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encrusting forams
1 60
4
Altematiny light--colored lime 1 r1 1 utJslone & da•ll shaly mudstone
40
& shale
1 20
Packstone, oohtic near top:
peloid s, tubulur fQrarns
Platy-algal limeslOTle tnaet mound): bafflestone, some
30 80
10 .
0
a:
Lime
0
UJ
substrate)
....J
Brown to black lime mudstone &
shal•\ s i lty. with broc:hiopod
40
& crinoid debris
0
R20W
R26E
6
T
40----�----�---4--���--+-��
T 35 N
�0
Hio4---,,:..j ' - \:J, --4---1 oo'l:
24
ur::J) /
Contour interval 20 ft
0
<:>'00
(:6 m )
Figure 3-Map showing wells cored in Ismay Zone and structure drawn on top of lower Ismay shale (B horizon in Fig. 2). Carbonate buildups are localized along axis of structure (from Choquette and Traut, 1 963).
Well cored i n Ismay zone 2 miles
0
Figure 2-Carbonate cycles conta i n ing two algal reef-mound build u ps i n the I smay Zone, Paradox Formatio n , in an oil-produc i ng well, I smay field. Well is the Pure Oil Company (Union Oil) East Aneth No. 28 D-1 in NW '.!.I NW '.!.I sec tion 28, T35 N , R26E. Cycles 1 and 4 contai n reef mounds and terminate in subaerial exposure here and in most other parts of the area. A t h i rd reef mound cycle, in the interval A2.5-A3 represented tlere by two inter-mound cycles, is d eveloped in many other parts of the area (see F i g . 6). Modified from Choquette and Traut ( 1 963).
/
/
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detritus is restricted to the dark shales and to scattered grains in the car bonate rocks.
GENERAL FEATURES OF THE
REEF MOUNDS Synthesis of the regional relation ships and general features of the mid Pennsylvanian carbonate build-ups
has been made by Wilson ( 1 975), who described some of the variations in geologic setting, distribution, shape and geometry, and lithology that have been reported by many workers on carbonates in the Paradox basin
456
Reef
Fi g ure 4-Map showi ng thickness of the A3-B i ntervaL Thick areas are areas of thick algal reef mounds. Thin channel l i ke areas contain spicule mudstone and fine-grained calcareous quartz sand stone. A3-B interval is mantled by dark shales and is ove rlapped by evaporites to the north and east {see Fig. 6) (from Choquette an d Traut, 1 963).
T
35 N
f
Figure 5-Map sh owin g thickness of the
A2.-A2..5 interval. Thick areas are areas
of th ick algal reef mounds. Reef mounds a re mantled by dark shales an d evapo ri tic dolomites g rading upward to, and overlapped by, evaporites (from Choquette and Traut. 1 963).
6
Carbonate build-up 10 It thick or more
Max. permeability 10 m
0
2 miles
. II� :JI6
7
,o I
34
3
10
Carbonate build-up
1 0 f1 thick o r more
32
3 Km
0
<{ ' a: r- i o
12
29
T 35 N
29
permeability 10 md or more
Max.
0
2
0
3 km
1---,---L-r---r...J
mile�
32
(Wengerd, 195 1 , 1 955, 1 963; Peter son, 1 959; Peterson and Ohlen, 1963 ; Pray and Wray, 1 963; Choquette and Traut, 1 963 ; Elias, 1 962, 1 963 ; Gray, 1 967; Hite and Buckner, 1 98 1 ) and the Orogrande basin (Wilson, 1967, 1972, 1 975). In general, the reef mounds appear to be flat-bottomed, convex-upward lenses with as much as 30 m of relief and regular to un-
dulatory tops. Pray and Wray ( 1 963) reported broad-scale cross bedding or accretionary bedding in biostromal banks of platy-algal grainstone which are exposed in the "Goosenecks" of the San Juan River, a series of large entrenched meanders about 1 10 km west of Aneth field. Although many of the build-ups are biostromal in the sense of having great lateral extent
compared with their thickness, slopes of 25 to 30° exist locally on the tops and flank edges of some reef mounds. The mounds may be capped by ooid-rich grainstone which is also an important reservoir lithology in many of the Paradox basin fields, and/or by grainstone and packstone containing abundant fusulinids and foraminifers or encrusting life habit (ophthalmidids); this latter facies also occurs at the margins of some mounds and may represent in part the more mobile debris derived from the mounds. Some mounds terminate up ward in platy-algal carbonates, at what are interpreted as erosional disconformities. Inter-mound facies are commonly fine-grained mudstone and wackestone rich in sponge spicules . Some reef mounds have been dolomitized but many remain limestone. A characteristic aspect of the reef mounds is the occurrence within them of breccias and con glomerates of algal-plate material and entrained fine carbonate sediment, suggesting partial induration followed by episodes of slumping or spalling of build-up sediments. Another characteristic feature is an association of reef-mound lithologies with multi ple, shoaling-upward cycles which begin at their bases with transgressive
Reel
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PURE OIL CO. E . .oi.NETH il2U[).. 1
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bo�mdstone. stromatoliti!> Pe!oi($e! snci/or tubulaf tornm packStone/g raiostooe Calcareous sand3tonc DnG! siltstone
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PAN AMERICA.N OIL CORP.
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shales and carbonate mudstones and culminate in peritidal or high-energy shoal carbonates or in exposure surfaces . Wilson ( 1 975) believes that the Des moinesian algal-plate reef mounds ac cumulated somewhat downslope (basinward) from "tectonically induc ed shoal s . " Paleoslopes seem to have been gentle, rarely more than 2 to 3 a , and successively younger reef mounds tend to overstep each other basin ward. Subtle relief on the sea floor may have been enough to cause the initiation and colonization of in dividual stands of leaf-like platy algae. Careful study of excellent ex posures along the San Juan River led Pray and Wray ( 1 973) to conclude that algal-plate biostromal bank se-
quences there developed by the lateral coalescing of more abrupt build-ups. REEF MOUNDS IN THE ISMAY FIELD: A CASE STUDY
A series of stacked algal reef mounds has been cored extensively in the Ismay field situated about 5 km northeast of Aneth near the edge of the algal-mound fairway outlined earlier (Fig. 1). These build-ups have been described by Elias ( 1 962, 1 963), and in detail by Choquette and Traut ( 1 963) from which much of the following sumary has been taken. Ismay is a relatively small field by comparison with Aneth and many other fields in the Paradox basin, having produced about 9.7 million
NE
Figure 6-Stratigraphic cross sections of Ismay Zone approximately parallel (A-A' ) and normal (RB ' ) to regional depositional strike (Figs. 4, 5). Well inter· vals here were cored in their entirety. N01e that the buildups are subtle, low· reli ef features when seen even at SX vertical ex1ll g geration. Sag of upper reef mounds in A-A' probably reflects dif ferential compaction of Inter-mound sediments, cored in the Pure East Aneth No. 278·3, 2 7 ·40S·26E (modified from Choquette and Traut, 1 963).
barrels of high-gravity �41 o API) oil and condensate, and 1 6.5 million cubic ft of low-sulfur gas through 1 978. Nevertheless, the reefmounds there have features in common with other mid-Pennsylvanian buildups in the southwestern United States as outlined earlier.
458
Reef
Figure 7 A-Platy-algal l i m estone. baf flestone of
Setting and Large-Scale Features The Ismay field reef-mounds occur in shoaling-upward, marine to peritidal cycles in three informally designated intervals of the Ismay Zone, Paradox Formation. The inter vals are limited by th� so-called A2, A2. 5 , A3 , and B horizons (Fig . 2). In the section summarized in Figure 2 only two reef-mounds occur, in cycles designated informally as 1 and 4. The rest of the cycles shown there are in terpreted as inter-mound sequences. We will return to the subject of these cycles after examining other aspects of the reef-mounds. The field itself lies along the axis of a northwest-plunging anticline as defined on the B horizon (Fig. 3). Available well control indicates that the reef-mounds are flat-bottomed , thinly lensoid, biostromal accumula tions up to about 1 5 m in thickness. Some of the buildups in the A3-B in terval are . 75 to 1 .5 km across and at least 3 km long, elongated northeast to-southwest almost at right angles to the anticline axis (Fig. 4). Build-ups in the two younger intervals of the Ismay Zone, illustrated by the isopach map of the A2-A2 .5 interval in Figure 5, are more extensive features some 2 to 3 km across and at
Figure 79-Micrite-rich platy-algal limestone ·with algal-mold POrosity (black) and patches of !ater po re - fi l l i ng anhydrite {while). Pure East Aneth No. 22D- 1 , 5793 ft near top of A3·B (cycle 1) reef mound. Exposure to meteoric water is suggested by the moldlc porosity; pl aty algae are inferred 10 have b�?.en aragonitic at the time.
Figure 7 D-Complex breccia. grainstone
(mudstone) of gray micrite fragments amj brown peloid·rictl fragments in a groundmass dominated by anhydrite (while). Both rBPiaoernent an<.l f.>Ort!·filfing
anhydrite occur. Most clasts are bordered by phylloid algae (a1rows), sug· gesting tl..,at they were d e_rived by break up (oft-mound slumping?) of partiy indu (ated sedrmen l during reef-mound con:>truction. Pure Aneth No. 280-1 , 5742 .4 ft. .., , . .... . ' . . .
least 9 km long northwest-to southeast. These mounds also display a "cross grain" nearly at right angles to the dominant trend. The cross grain may result from some combina-
Figure 7C- Ool o m it!zed platy-algal baf· flestone. Phylloid argae (dark) entrapped lime mud (now micrite, pale tan} and peloids, now peloid g rainstone and packstone. Diagenetic infill and partial replacement by anhydrite destroyed primary porosity. Pure East Aneth No. 280-i , 5742 ft, from the upper (A2·2.S) reef mound i n tills well.
tion of tidal currents and prevailing winds. Given the regional paleogeography (Fig. 1), it seems like ly that tidal currents swept back and forth across the margin of the basin. Recently published plate-tectonic reconstructions (Scotese et al , 1 979) suggest that in mid-Pennsylvanian time the basin was situated near the paleoequator . Comparison with pres ent global wind patterns suggests that there may not have been strong prevailing winds in the region at that time. Reef-Mound Facies In cross sections (Fig . 6), the flat bottomed form of the reef mounds is apparent for the oldest (A3-B) build ups. The undulose appearance of the upper two build-ups in section A-A ' may in part be a result of compaction draping over the lower reef-mounds. The dominant lithology of the reef mounds is algal bafflestone (Embry and Klovan, 1 97 1) in which loosely to closely packed fragments of the leaf like or phylloid codiacean alga Ivanovia were the "constructing" elements. Pray and Wray ( 1 963) pro vided an interpretation of the prob able life form and the sediment baffling , sediment-trapping effects of
Reef 459
Figure 8A-Dark lime m udstone with pelmatozoan debris and signs of biotur bation, from t h e basa( "8hale " unit of cycle 1 (Fig. 2), Pure East Aneth No . 280·1 , 583 1 ft.
Fig ure 8 8-S h e l ly lime mudstone, the usual subgtrate on which reef-mound deposition bega n . Some brachiopods were broken by compaction. Dolomitiza tion in places of this substrate facies created good reservoir rocks with inter· crystal and biom o ldic porosity. Core is from cyc!e 1 (A3·8 interval) in Pure East Aneth No. 28D·1 , 5820. 5 ft.
this alga. Ivanovia was probably an upright leaf-like marine plant which became calcified and upon death broke into somewhat rubbery, stiff but not rigid, platy fragments. Because of their broad, curved shapes, these fragments served as sediment traps in which lime mud, peloids and other particles collected (see Fig. 7 A and 7C). The platy fragments also served as sheltering "umbrellas" beneath which shelter voids stayed free of sediment. Some of these original pores survived com paction and cement infill ; others were filled by precipitated cements com posed mainly of blocky calcite spar and/or coarse anhydrite (see Fig. 7B, Figure 80-Burrowed. organic-rich , dolomit ized l ime mudstone . This lithology 7C, SA, 8B). Variations of bafcommon:y occurs at or near the tops of flestone fabrics with partly filled or more �vaporiti�. off-mound cycles. It is occluded interparticle and shelter o�e rla1n gradationally or abruptly by euxporosity are well illustrated by Cho101c ?arbonate mudstone. Pure EaS1 quette and Traut ( 1 963) . Other biotic constituents of the reef Anetn I'Jo. 28 0 · 1 5743 . 4 ft . mounds i n addition t o Ivanovia include rare fenestrate bryozoans which also serve as sediment baffles, imentary origin are common as noted pelmatozoan debris, an occasional earlier. Typically they consist of fusulinid, ostracodes, and angular fragments of lime mudstone ophthalmidid foraminifers. The oph and/or peloid packstone or thalmidids rarely occur in life posi grainstone bounded on one side by a tions encrusting Ivanovia fragments phylloid algal plate (Fig . 7D) . The and more commonly occur as loose nature of these clasts suggests only skeletal grains. Peloids and micrite are the only common non-skeletal partial induration when the breccias accumulated . The nature of the brec constituents. cias suggests that some combination Reef-mound breccias of synsed-
Figure 8C-Pelo id-fusu linid packstone capping the A3·R (Gycle 1 ) algal reef· mound in Pure East Aneth No. 280·1 , 5782 ft . The smaller grains are peloids and p el o id molds (black); the larger grains are fusulinids and lithoclasts. Subaerial e>-..posure of the reef-mound "cap'' at the close of cycle 1 (Fig. 2) probably callsed the so l u t ion porosity.
of break-up by storms and off-mound slumping may have been responsible for them . Reef-Mound and Associated Cycles The thickest and most complete se quence in which Ivanovia reef mounds are developed in the Ismay area is in the lower part of the Ismay Zone referred to as cycle 1 in Figure 2. This sequence begins with a widespread unit of black shale and dark grayish-brown lime mudstone containing scattered crinoid colum nals and phosphatic brachiopods, the so-called "B Shale" (Fig. 8A) . This lithology suggests anoxic marine con ditions across a broad but not necessarily deep basin of rather low relief. It grades upward to light colored lime mudstone (or wackestone in places) of less anoxic origin containing a more normal marine but still sparse fauna, or thin shelled calcitic brachiopods, scattered crinoid debris, and scattered ramose fenestrate bryozoans . This light colored muddy lithology which Cho quette and Traut ( 1 963) termed "shel ly calcilutite," is the substrate upon which the algal reef mounds of cycle 1 became established or colonized. An example of the lithology is shown
460
Reef
Figure 9A-Photomicrog raph of calcite cements in Ivano via grainstone (plane light). Fi n e ly crystalline radiaxial cement (arrows) of ma rine o rigi n borders ex terior of phylloid alg�e (dark). Coarser blocky calcite cement fills the remaining pore spaces between algal fragments as well as interiors of algae . Original porosi ty was probably more than 70rtro . Large white areas at lef1 and lower right are artifacts, as are the cracks. Texaco Navajo No. J - 1 , 20-40S-26E, 5623 ·24 ft.
Figure 98-Same J.>t1otornic rograph as A, in cross-polarized l ight . Fringes of radiaxial cem e nt are more clearly ap parent here.
in Figure 8B and 8C. The upward change from this lithology to algal carbonates of the reef-mound facies is variable but generally rather abrupt over a few millimeters or centimeters, and in fact is often obscured by stylolites (Fig. 8C). Figure 9D-Photomicrograph of a l ga l Carbonate "sand," either packstone (plane light). Coarse blocky packstone or grainstone, generally calcite fi l l s former molds of calcareous caps the cycle 1 reef mounds as well (possibly aragonitic) phylloid algae as younger reef mounds (A-A ' in Fig. (genus unknown bu1 possibly Ivanovia). 6). This lithology typically contains Pure East Aneth No. 27C-4, 27-40S-26E, fusulinids, ophthalmidids, peloids, near base of A2 ·A2.5 build-up, 2445 ft and occasional fragments of phylloid algae, bryozoans, and brachiopods. Rarely, small heads of chaetetid corals occur. This lithology is similar to the "cap facies" of Pray and Wray Capping carbonate sands spill downslope for short distances of I ( 1 963). It also resembles in most km or less from the reef-mound respects the ' 'oolite' ' capping facies flanks, and give way to muddy of Peterson and Ohlen ( 1 963) at spiculite and siliclastic siltstone and Aneth, a lithology later redescribed by Wilson ( 1 975) as rich in "tubular" fine-grained sandstones. These lithologies also fill shallow (ophthalmidid) forams and leached "channels" between mounds (A-A' "oolites. " Leached-grain molds are in Fig. 6) and extend short distances indeed common in the capping facies into the basin (B-B ' in Fig. 6). The of cycle 1 (Fig. 8C) and were pro capping carbonate sand is overlain bably aragonitic when they were abruptly, at an apparent disconformidissolved; they may have been ooids.
Figure 9C-Photom icrograph of Ivano via g rains1one {plane l i g h1). The phy!loid algae are better prese rved than in A o r B. Darker material in upper left quadrant is peloidal silt. Most cement is fine blocky calcite, but the white areas (arrows) are occupied by anhydri!e ce ment. Pu re East Aneth No. 27A-2, 27·408·26E, A3·B bui ld-up, 5679 ft.
ty, by dark crinoidal lime mudstone interpreted as the basal anoxic unit of the next younger cycle (cycle 2 in Fig. 2). Younger reef mounds commonly terminate upward in, and may also contain intervals of, laminated peloidal limestone (or dolomite) con taining mudcracks, mud-chip con glomerates, fenestral fabrics, and other products of deposition on high intertidal flats that were periodically exposed. These tidal-flat lithologies are in some areas overlain by dolomitized mudstones, some of which are organic-rich and burrowed (Fig. 8D), and others having abun dant nodular anhydrite. The cycle in which lithologies like those just described occur are more evaporitic than the "ideal" reef mound cycle outlined earlier (Fig. 2, A3-B buildup). Similar evaporitic cycles can be seen in areas immediate ly basinward from the reef mounds. There the cycles contain thicker units of dark shale and carbonate mudstone, evaporitic dolomite, and massive to bedded anhydrite. Units of anhydrite in these cycles thicken into the basin, in a general northeasterly direction (Fig. 6). Non-evaporitic se-
Reef
quences in areas between the reef mounds but along the reef-mound trend itself generally are thin. They begin with the lower shale and substrate mudstone units found in reef-mound cycles, but these are overlain by rather dark-colored mudstones made up of sponge spicules and other fine-grained skeletal debris together with micrite. Thin units of siliciclastic siltstone and fine to very fine grained sandstone also occur in some inter-mound cycles , particularly in the lower Ismay Zone (Fig. 6, section A-A ' ) . The siliciclastic beds may be products of lowered sea level during which fine detritus reached the basin from land to the south. Diagenesis and Porosity of Reef-Mounds As noted earlier, some cycles in which algal reef mounds were developed apparently ended in subaerial exposure. Disconformities occur at the tops of these cycles (see Fig . 2). The limestones in these cycles show signs of dissolution which was selective only to some of the original grains, presumably those that were aragonitic . Moldic porosity is quite common, particularly in lime-mud rich facies of reef mounds where phylloid algae were selectively dissolv ed (Fig. 7B), and also in capping limestones where ooids and possibly aragonitic peloids(?) were dissolved (Fig . 8C) leaving micrite or early ce ment intact. Molds like these in the reef mounds are commonly filled, however, by blocky calcite of phreatic origin (Fig . 9D). Cement-filled moldic porosity is more common in the lower parts of reef mounds than in their up per parts or in capping limestones , suggesting that the cementation by blocky calcite may have begun early, perhaps before deposition of the next cycle. Phylloid-algal grainstones have at least two generations of carbonate cement - an earlier one of fine radiaxial calcite in fringes up to 0.2 to 0.3 mm thick along the exteriors of algal plates (Fig. 9A, 9B), and a later phase of coarse, blocky calcite which filled or partly filled remaining pore space (Fig. 9A to 9D). The radiaxial calcite is previsionally j udged to be of marine origin . Examination in
cathodoluminescence of a few thin sections of grainstones cemented by blocky calcite has shown the calcite to have multiple zones of varying luminescence. This suggests that detailed study in cathodoluminescence would reveal a more complex cemen tation history perhaps relatable to repeated exposure as successive cycles accumulated . Coarse anhydrite cement is also common in the reef-mound car bonates, especially the grainstones and open-work breccias (Figs. 7C, 7D, and 9C), in places completely oc cluding both original porosity and solution porosity. This evaporite ce ment post-dates the blocky calcite. It may be a result of sulfate reprecipita tion from waters circulated through evaporite cycles in areas basinward from the Ismay area, or in evaporites overlying the Ismay field reef mounds . More focused study of diagenesis in the reef mound and associated rocks is needed to test this possibility and alternative models. Also poorly understood is the distribution and origin of dolomitiza tion which has affected parts of reef mounds and underlying substrate lime mudstones in places enhancing the reservoir-rock quality of both (Cho quette and Traut, 1963). CONCLUSIONS
Reef mounds in which platy or phylloid algae were important sediment-constructing organisms are significant features of mid Pennsylvanian cyclic carbonate se quences in the southwestern United States. In the Paradox basin, algal reef mounds occur mainly in a north west-trending " fairway" (Wilson, 1 975) up to 40 km in width and 85 to 90 km in length along the southwest margin of the basin (Fig. 1). Many of these reef mounds are reservoir rocks for oil and gas, with reserves ranging from small, as in Ismay field, to giant, as in Aneth . The reef mounds are biostromal in dimensions, with maximum thicknesses of about 10 m and areal extents ranging up to a few kilometers in any one direction. They consist predominantly of algal baf flestone (grainstone-packstone) in which loosely packed semi-rigid plates
461
of Ivanovia baffled and entrapped carbonate sediment, creating high original shelter and interparticle porosity, and synsedimentary breccia composed of algal-limestone in traclasts. Phylloid-algal wackestone and packstone with abundant micrite is also abundant, particularly in the lower parts of reef mounds. The brec cias appear to be the only significant "reef-flank" material and are so closely associated with in-place baf flestone that they have not been map ped separately . Grainstone and packstone carbonate sand composed mainly of fusulinids, ophthalmidid foraminifers, and peloids is a com mon capping lithology on the reef mounds. Reef mounds occur in asym metrical, shoaling-upward cycles 30 to 40 m thick which began with widespread basal dark shales and lime mudstones of anoxic marine origin, and terminated with foram-peloid ooid carbonate sands of shallow marine to beach or possibly dune origin, or peritidal carbonates. Subaerial exposure was common at the ends of cycles . Much of the diagenetic modification of reef mounds involved early selective leaching of aragonitic grains (vadose?) and later partial to com plete cementation by block calcite (phreatic?) and/or coarse anhydrite. Parts of many reef mounds were also dolomitized.
ACKNOWLEDGMENTS The author acknowledges the en couragement of Noel P . James, Memorial University of Newfoundland, at whose suggestion this summary of earlier work by John D. Traut and myself was written. Susan C. Hartline and other members of the Illustrations Department, Marathon Oil Company Research Center, prepared the illustrations. Published by permission of Marathon Oil Company.
SELECTED REFERENCES Choquette, P. W. , and J . D. Traut, 1963, Pennsylvanian carbonate reservoirs, Ismay field, Utah and Colorado, in R . 0 . Bass and S. L . Sharps, eds . , Shelf carbonates o f the Paradox basin, a symposium: 4th Field Conf. , Four
462
Reef
Corners Geol. Soc. , p. 1 5 7- 1 84. Elias, G. K., 1962, Paleoecology of lower Pennsylvanian bioherms, Paradox basin, Four Corners area: Guidebook, 27th Ann. Field Conf. , Kansas Geol . Soc . , p. 1 24- 128. ___, 1963, Habitat of Pennsylvanian algal bioherms, Four Corners area, in R. 0. Bass and S. L. Sharps, eds . , Shelf carbonates of the Paradox basin , a symposium: 4th Field Conf. , Four Corners Geol. Soc . , p. 1 85-203 . Embry, A. F . , and J. E. Klovan , 1 97 1 , A Late Devonian reef tract on north eastern Banks Island, Northwest Ter ritories : Canadian Petroleum Geols. Bull . , v . 1 9 , p . 750-78 1 . Gray, R . S. , 1967, Cache Field-a Pennsyl vanian algal reservoir in southwestern Colorado: AAPG Bull . , v. 5 1 , p. 1 959- 1 978. Hite, R . J . , and D . H . Buckner, 1 98 1 , Stratigraphic correlations, facies con cepts and cyclicity in Pennsylvanian rocks of the Paradox basin, in D. L . Wiegand, ed . , Geology of the Paradox basin : Rocky Mtn. Assoc. Geols., p. 147- 1 60.
Krivanek, C. M . , 1 98 1 , New fields and ex ploration drilling, Paradox basin, Utah and Colorado, in D. L. Wiegand, ed . , Geology o f the Paradox basin: Rocky Mountain Assoc. Geols . , p. 77-8 1 . Peterson, J . A . , 1 959, Petroleum geology of the Four Corners area: Proc . , Fifth World Petroleum Cong . , Sec. 1 , Paper 27' p. 499-523. __ and H . R. Ohlen, 1 963 , Pennsyl vanian shelf carbonates, Paradox basin, in R. 0. Bass and S. L. Sharps, eds . , Shelf carbonates o f the Paradox basin, a symposium: 4th Field Conf. , Four Corners Geol. Soc . , p. 65-79. Pray, L. C . , and J . L. Wray, 1 963 , Porous algal facies (Pennsylvanian), Honaker Trail, San Juan Canyon, Utah, in R . 0 . Bass and S . L. Sharps, eds . , Shelf carbonates o f the Paradox basin, a symposium: 4th Field Conf. , Four Corners Geol. Soc . , p. 204-234. Scotese, C. R . , et a!, 1 979, Paleozoic base maps: Jour. Geology, v. 87 , p. 2 17-277. Wengerd , S. A . , 1 95 1 , Reef limestones of Hermosa Formation, San Juan Can yon, Utah: AAPG Bull . , v. 35, p . 1038- 1 05 1 .
, 1 955, Biohermal trends in Penn sylvanian strata of San Juan canyon, Utah, in Geology of parts of the Paradox, Black Mesa, and San Juan basins: Field Conf. Guidebook, Four Corners Geol. Soc . , p. 70-77. ___, 1963, Stratigraphic section at Honeker Trail, San Juan Canyon, San Juan County, Utah, in R. 0 . Bass and S. L. Sharps, eds . , Shelf carbonates of Paradox basin , a symposium: 4th Field Conf., Four Corners Geol . Soc . , p . 235-243. Wilson , J. L . , 1 967, Cyclic and reciprocal sedimentation in Virgilian strata of southern New Mexico: Geol. Soc. America Bull., v. 78, p. 805-8 1 8 . ___ , 1 972, Influence of coral structure in sedimentary cycles of Beeman and Holder formations, Sacramento Moun tains, Otero County, New Mexico, in J . C . Elam and S. Chuber , eds . , Cyclic sedimentation in the Permian Basin, 2nd ed . : West Texas Geol. Soc . , p . 4 1-54. ___, 1 975, Carbonate facies in geologic history: New York, Springer-Verlag Pub . , 471 p. ___
464
Bank Margin
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Figure 1 -Bank·m.argln sands (h igh·
lighted in yellow) play an important part i n geologic models, whether deposi1ion occurred over a shelf-margin or a ramp
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arbonate sand accumula tions of reservoir size com monly occur on or near the seaward edge of banks, platforms and shelves. They may also form within the platform interiors or on topographically high areas in regionally deep water, but these oc currences are not as common as those along the margins. Bank-margin sand accumulations may grade landward or
seaward within a fraction of a kilometer of other environments and, thus, do not have wide lateral extent in a dip direction . Such accumula tions are sufficiently distinct and economically important as carbonate reservoirs to warrant their treatment in some detail. This paper provides an overview of modern and ancient carbonate sand bodies, and refers the reader to detailed work covering
orotile.
many aspects of carbonate sands. Bank-margin carbonate sands occur repeatedly throughout the geologic record and are a prominent compo nent of carbonate facies models (Fig. 1 ). Shaw (1 964) and Irwin ( 1 965) recognized the persistence of this
Bank Margin
SKEU:TAL
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465
74
CAABOit�TE SED IM ErlTS GAAif,ISTONE WACKESTONE
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Figure 2-Location map and sediment distribution map for south Florida, and
the Bahamas by E nos
(1974). Note l t H:lt
skeletal grainstones, oolitic grainstones,
and mixed oolitic-pelletoidal g rai nstones (shown in shades of green) rim most of
the carbonate platforms.
facies in epeiric sea models as a seaward high-energy zone separating low-energy, deeper-water sediments from low-energy, shallow-water lagoonal deposits. More recent models by Heckel ( 1 972), Lees ( 1 973), and Wilson (1 975) emphasize the im portance of the zone in which car bonate sands accumulate. In nature, sand accumulations are not distinct and isolated from other facies (as are the chapters of this volume from each other, for example), and there will necessarily be some overlap between this chapter and those concerning reefs , beaches and islands, and lagoons.
Our understanding of carbonate sand deposition is biased toward bank-margin deposits because most modern studies have focused on these. The distribution of bank margin sands in the Bahamas, where most studies have taken place, is well illustrated on a map showing sedi ment distribution (Fig . 2). It is temp ting to interpret ancient deposits to fit a modern example, but we must re mind ourselves that the Bahamian ex amples are not representative of all accumulations in the geologic record . In this review, most modern observa tions come from the Bahamas, but readers are encouraged to review those references to carbonate sand studies from other areas . These studies broaden our concepts of the variability of carbonate sand deposits. For example, the Persian Gulf (Ara bian Gulf) provides an excellent modern example of carbonate sands
deposited on a gently sloping shelf or ramp (Ahr , 1 973), but these deposits have received much less study than Bahamian sands. PREVIOUS WORK
Before the turn of the century, most research on modern carbonate sediments centered on reefs. After 1 900, other types of carbonate ac cumulations were examined in in creasing detail. These studies are sum marized in part by Illing ( 1 954). By 1 950, the ground work had been laid for more detailed investigations of carbonate sands, with studies sup ported by strong interests from both academia and the petroleum industry. During the 1 950s and early 1 960s , carbonate sand studies emphasized particle analyses and morphology of sand accumulations. The composi tion, origin, size, shape, and distribu-
466
Bank Margin
TABLE 1 Skeletal
Grains
Non-Skeletal Grains
calcareous algae mollusk
ooids fecal pellets
foraminifera
lumps grains
coral
aggregates
Table 1 . Major types of carbonate sand g rains (modified from l l l i n g , 1 955).
F ig u re 3-Gemini photograph of the south end of Tongue of the Ocean (TOTO) and Exuma Sound in the Bahamas. A tidal bar belt rims southern TOTO and the northeastern rim of Ex· uma Sound. The field of view ac ross the base of the photog raph is about 1 20 km .
tion of carbonate sands were documented over wide areas of the Bahamas and southern Florida. M uch of this work was the result of projects undertaken by Norman D. Newell, Leslie V . llling, Robert N . Ginsburg, and their colleagues and students. Ry the late 1 960s and early 1 970s, the emphasis had moved to internal struc ture of the sand bodies and early diagenesis of sand grain s . Leaders in these aspects of carbonate sand studies were John Imbrie, Edward G . Purdy, Mahlon M . Bal l , H ugh Buchanan, and Paul Enos. Studies entered another stage in the late 1 970s, concentrating on the hydrodynamics of carbonate sand movements (Hine, 1 977; Harms et a!, 1 978). If these i nquiries conti nue , they should provide a firm basis for comparisons between carbonate and siliciclastic sand deposits.
Carbonate Sands Illing ( 1 954) proposed a subdivision for calcareous sands into categories of "skeletal" and "non -skeletal" (Table 1 ) . The reader will be familiar with most of the subdivisions, with the possible exceptions of the non s keletal grain types - grains, lumps, and aggregates . Grains are aragonitic, micritic grains of indeterminate origin. Aggregates are weakly cemented clumps of sand grains that can easily be separated . Lumps are well-cemented grain-aggregates that include grape-li ke clusters (grapestone), botryoidal lumps, en crusted lumps, and irregular forms. Purdy ( 1 963) and Winland and Mat thews ( 1 974) have shown that no genetic distinction exists between ag gregates and lumps. Numerous photomicrographs of these and other carbonate grain types have been illustrated by M illiman
( 1 974), Bathurst ( 1 975), and Scholle ( 1 978). Unfortunately , some of the criteria by which modern non-skeletal grains are recognized are not ap plicable to ancient carbonate rocks . McKee and Gutschick ( 1 969) have suggested the term "peloids" to em brace all cryptocrystalline grains of uncertain origin . Although this term is admittedly a label of ignorance. it has proven an extremely useful term in the description of ancient limestones . The grain types of Illing ( 1 954) are now recognized to com prise over 9007o of the grains in modern shallow-water carbonate sands. Besides the four skeletal grain types listed by Illing, other organisms contributing to carbonate sands are echinoderms, brachiopods, ar thropods, annelids, byrozoans , and sponges . Of these, echinoderms, brachiopods, and arthropods were conspicuously more important sand
Bank Margin
producers during the Paleozoic. MODERN SAND BODIES
Recent studies have shown that modern carbonate sand bodies associated with bank, platform, or shelf margins reflect the orientation, exposure, and topographic complexity
of those margins. Winds, waves, cur rents, tides, biogenic barriers, and rock ridges or terraces along specific margins control sediment type, sedi ment production, sediment transport, and sand-body geometry, orientation, and size. Because knowledge of these last three characteristics is paramount to subsurface paleo-environmental
467
Fig u re 4-Landsat photograph of the Schoone r Cay t idal-ba r belt, r i m m i n g northeastern E x u m a Sound in the Bahamas. The abrupt change to deeper water is clearly shown by a color c hange. The field of view across the base of the photograph is approximately 65 k m .
468
Bank Margin
rmuo nm BA1 CREST
STRIK£ SECTION OF CROSSI£DS II Mill ME ORIGINAl SINO BHI
Figure 5-Schematic block diagram from Ball ( 1 967) showing sedimentary structures that typify tidal-bar belts /Ike those rimming southern Tongue of the Ocean and northeastern Exuma Sound.
Figure 6-Aeria/ photograph of tidal bars at southern rim of the Tongue of the Ocean. The abrupt change in water col or toward the left side of the photograph reflects rapidly increasing water depth. The individual tidal bars are long (up to 20 km), narrow (1 to 1 .5 km wide) and covered with smaller sand waves.
analysis, our presentation of modern carbonate sand bodies will follow Ball's ( 1 967) classification stressing geometry and orientation rather than adopting a scheme based on some other factor, such as dominant physical process or bank-margin ex posure. Ball (1 967) classified sand ac cumulations of the Bahamas and south Florida into four groups: ( I ) tidal-bar belts; (2) marine sand belts; (3) aeolian ridges (dunes); and (4) platform interior blankets. The dunes and platform interior sand bodies are discussed elsewhere in this volume, so
they will not be stressed here. The emphasis here will principally be on the wide variety of tidal bar and marine sand belts that develop along carbonate bank margins. The primary difference between tidal-bar and marine sand types of linear sand belts is their orientation with respect to the trend of the bank margin . Tidal-bar belts develop perpendicular to the margin, whereas marine sand belts form parallel to the bank edge. Both types of belts res pond primarily to daily tidal flows, along with wave and storm-generated currents; but significant differences
exist between them, as well as be tween individual sand accumulations within each sand belt. Such dif ferences result from antecedent topographic control, response to sea level change, response to storms, bed form distribution, role of diagenesis, development of benthic communities, sediment type, sediment thickness, and lateral facies changes . The critical factor determining whether a sand body develops into a tidal-bar belt or a marine sand belt is the dominance of daily tidal flows . When peak tidal currents are high ( > 100 em/sec), parallel-to-flow sand
Bank Margin
469
F igu re 7-Aerial photograph showing detail of tidal bars from Fig u re 4 . Light colored ooid sands mark the active por tions of the tidal bars. The bars are sepa rated by deeper channels, also cur rent swept but commonly covered with g rass o r cemented-sand bottoms.
Figure 8-Aerial photograph showing detail of tidal bars from F i g u re 6. Transverse sand waves, oriented approx imately 45° to the length of the tidal bar, adorn the nearly intert idal, highly agitated sea bottom of clean ooid sands.
bodies develop . When peak tidal cur rents are not as great (50 em/sec) but still well above critical threshold levels, a sand belt normal to the tidal flow and parallel to the bank edge can develop. Along margins where high tidal currents form tidal-bar belts at or near the bank edge, these same currents are reduced further on the bank top by frictional effects. As a result, a bank -normal tidal-bar belt may merge into a bank-parallel
marine sand belt. Some other modern sand accumula tions along bank, platform , or shelf margins do not occur as belts. Tidal deltas, lobate in form, are generated by tidal currents flowing through gaps between islands. Also, back-reef sands may migrate into lagoons as lobate forms. Finally, there are com posite sand bodies, which exhibit at tributes of several end member types .
TIDAL-BAR BELTS
The classic examples of carbonate tidal-bar belts are located along the southern end of the Tongue of the Ocean (Fig. 3), and the north end of Exuma Sound (Schooner Cay area; Fig. 4), on Great Bahama Bank, Bahamas. The tidal-bar belt along Tongue of the Ocean dominates the bank-margin for nearly 100 km; the tide-dominated zone along Exuma
470
Bank Margin
Fig u re 9-Uilderwater photograph of the lee slope of a mobile sand wave located in a tidal channel near Joulters Cays o n G reat Bahama Bank. The slip lace is about 1 m high and the water depth is 4 m .
Figure 1 0- U nderwater photograph showing detail of the lee slope of a sand ·wave like that of Fig u re 9 . Note the sort· ing produced down the avalanche s lope (I mbrie and Buchanan, 1 965) as larger g rains cascade to the base of the bed form and b u ry rippled ooid sands. The contact along the base of the slope is not straight.
Sound is nearly 45 km wide. These sand bodies have been studied and il lustrated by Newell ( 1 955), Newell and Rigby ( 1 957), Ball ( 1 967), Dravis (1977), and Palmer ( 1 979). The sedimentary structures and bar mor phology of tidal-bar belts are sum marized schematically on a block diagram (Fig. 5). Specific tidal-current data have not been published for these two areas , but daily tidal currents surely
dominate these bank-margin settings. Charts indicate that velocities of up to 200 em/sec occur. Also, these sand bodies are similar in geometry and size to siliciclastic sand bodies found on known tide-dominated shelves such as the embayments surrounding the North Sea (Off, 1 963 ; Houbo1dt, 1 968; Caston, 1 972; Hayes, 1 979). The north end of Exuma Sound and south end of Tongue of the Ocean are open (no island rims) em-
bayments . The setting augments the tidal range, thus further increasing bank-normal tidal currents at the margin (Southard and Stanley, 1 976). The individual tidal bars within the tidal-bar belts range from 0.5 to 1 . 5 km in width, 12 to 20 km in length, and 3 to 9 m in thickness (Figs. 6, 7). Tidal bars are separated by I to 3 km wide channels dominated by tidal cur rents. The channels are 2 to 7 m deep and are generally floored by sea
Bank Margin
471
Figure 1 1 -Aerial photograph of a small portion of the Cat Cay marine sand belt along the western margin of G reat Bahama Bank. From bottom to top, the color changes reflect water depth and bottom vegetation: grass-c overed, dark, shallow-subtidal sands; white, h ighly ag itated, nea rly intertidal ooid sands; and rapidly increasing water depth toward the bank-marg i n . The broad shoal to the left of the prominent tidal channel is about 1 km wide.
Figu re 1 2- Landsat image of the Berry I slands, n orthern Great Baha ma Bank. The open bank-margin to the n ort h and west permits vig o r ous tidal exchange w h ich is sufficient t o maintain the sand belt . Field of view is 90 km.
grasses and algae. Despite the high tidal-current velocities, there are only a few places where sand waves (ter minology of Boothroyd and Hub bard, 1975) are active on the floors of these channels. Sand waves are the dominant bed forms covering the tidal bars themselves (Fig . 8). The axes of the bars support large symmetrical sand waves whose crests are oriented obli que to or roughly parallel to the bar axis. Smaller-scale mega-ripples and ripples dominate the flanks of these
bed forms. These sand waves indicate little net transport and probably sup port slipfaces which reorient themselves as the tidal flow changes direction . This bed form orientation also indicates that flow on the tops of the tidal bars is oblique or normal to their trend. Even though the great mass of tide-driven seawater flows parallel to the bars, the currents near the bars are refracted toward their crests, thus developing the observed bed form orientation and distribu tion. In places , smaller channels have
actually cut through these sand bodies, resulting in the development of lateral bulges or spillover lobes (Ball, 1 967). Along the flanks of the tidal bars in slightly deeper waters, upslope oriented, asymmetric sand waves pro vide sand to the bar crest (Figs. 9, 1 0) . This bed form distribution in dicates that the channels on either side of the sand bodies may be flood or ebb dominant. Tidal-current segregation and resulting time-velocity asymmetry is important to the
472
Bank Margin
Fig u re 1 3-Satell ite photograph of the northern rim of Little Bahama Bank from Hine ( 1 977) highlighting the Lily Bank marine sand belt and other sedimen tologic features. Note the linear sand ridges adjacent to the marine sand belt.
development and maintenance of these sedimentary units _ Storm effects on tide-dominated sand bodies such as tidal-bar belts are largely unknown. The small channels that cut across individual bars and the development of spillover lobes may be storm-dominated events. Variations in surface sediments and biota have been well documented for the bank-margin surrounding Baha mian tidal-bar belts . Seaward of a tidal-bar belt, the sediment changes abruptly from ooid sands to a mix ture of skeletal, peloidal, and ag gregate sands. In this area, the sea bottom is not highly agitated due to greater water depth and supports a
variable covering of sea grasses and algae. A similar flora is also present bankward of the ooid bars, but the sediments are pellet sands or mud and are intensely burrowed . Sediment changes in the bankward direction are generally more gradual than those in a seaward direction. Other Examples Other tidal-bar belts in the Bahamas include portions of the J oulters ooid shoal and the Frazers Hog Cay area, north of Andros Island on Great Bahama Bank, and linear sand ridges adjacent to Lily Bank on the northern margin of Lit tle Bahama Bank. These sand ridges, up to 5 km long, 2 to 5 m in relief, and 200 to 600 m apart, are much smaller than those along Tongue of the Ocean and Exuma Sound. They are now partially covered with sea grasses and are thought to be relict,
perhaps being active only during large hurricanes. Surface sands are mostly lumps and micritized ooids with some peloids and skeletal fragments. The orientation of the bars, roughly nor mal to the bank edge (actually radiating away from a reentrant), in dicates that they were at one time responding to tidal and storm generated flows . Another tide-dominated zone oc curs between Grand Bahama and Great Abaco Islands on Little Bahama Bank. Although smaller in size, these sand ridges are similar to those along Tongue of the Ocean and Exuma Sound. The ridges respond to both ebb and flood currents as shown by the sharply defined symmetrical and asymmetrical sand waves on their crests and flanks. An example of a carbonate tidal bar belt not located in the Bahamas is the linear sand-shoals formed along
Bank Margin
473
Figu re 1 4-Schematic block diag ram
from Ball (1 967) i nd ica ti ng sedimentary structu res and grain types that typify m ar i ne sand belts like those along western Great Bahama Bank and no rth ern Little Bahama Bank.
the Trucial Coast, offshore of Abu Dhabi in the eastern Persian Gulf. Ooid shoals are located between off shore islands and the mainland where tidal flows are strong (Loreau and Purser, 1973). There are numerous other modern examples of small , linear, parallel-to-flow carbonate tidal-bar belts, particularly in areas where the tidal range is high. However, the Bahamian examples re main the largest and best documented of modern analogues . MARINE SAND BELTS
Marine sand belts, those linear sand bodies oriented roughly parallel to the bank, platform, or shelf edge, demonstrate a much greater degree of variability in geometry, size, sediment type, and origin than do tidal-bar belts. They occur both along wind-
ward as well as leeward bank-margins and may consist of skeletal or non skeletal sands. Three well-documented marine sand belts occur in the Bahamas: (1) the Cat Cay ooid shoal lying south of Bimini along the western margin of Great Bahama Bank (Purdy, 196 1 ; Ball, 1967; Fig. 1 1 ); (2) the platform behind the Berry Islands (Buchanan, 1 970; Hine et al, 1 98 1 ; Fig. 1 2); and (3) Lily Bank on northern Little Bahama Bank (Hine, 1 977; Fig. 1 3). Figure 14 is a schematic block diagram that summarizes the sedimentary structures and bar mor phology of sand belts. These marine sand belts range from 1 to 4 km in width and from 25 to 75 km in length. They are maintained by daily tidal flows, but each is also significantly influenced by storms (Perkins and Enos, 1 968; Hine, 1 977). Storm-generated flows may be responsible for developing channels normal to the axis of the sand body. These short channels are either floor or ebb dominated and commonly ter minate in spillover lobes (Fig. 13).
The dominant bed forms are sand waves (both symmetrical and asym metrical) with spacings ranging from 1 0 to 1 00 m and heights of 0.5 to 4 m. Smaller-scale ripples and mega ripples are ubiquitous on the stoss slopes of the sand waves . The crests of some of the sand waves reach within 0.5 m of low tide. As in the case of tidal-bar belts, important sea bottom and sediment changes have been documented for the bank-margin adjacent to marine sand belts. In a seaward direction, the sediments are a mixture of pellet, ag gregate, and superficially coated ooid sands with an admixture of skeletal grains. The bottom is stabilized by sea grasses and green algae. On the bankward side of the sand belt, grasses and algae cover a burrowed bottom of lime mud or muddy peloid and superficially coated ooid sand. Both the seaward and bankward sedi ment changes occur rather abruptly at the margins of the sand belt. Other Examples Along open, leeward margins in the
474
Bank Margin
GRAND BAHAMA ISLAND
�·
15 m � · ----�----------����·��
Ill HOLOCENE REEF Ill PLE I STOCENE ROCK Fig u re 1 5-Seismic profiles l o c ated seaward of Grand Bahama Island on t le Bahama Bank. Holocene ske l eta l sands have been transported off the shallow bank and trapped in de epe r
0
500m
Lit
water behind reefs. Accumulations up to 7 m thir.k hi'IIIP. fnrmArl
Figure 1 6-Ae rial photograph of Escbl l o de Arenas sand belt off V i eg ras I sland east of Puerto Rico. N ote the numerous sand waves on top of the shoa l . Puerto Rico is in the backg round.
Bahamas, carbonate sands originating in the bank interior may be transported to the margin, where they accumulate to form bank-parallel sand belts. Lumps, peloids, and
micritized ooids, along with algae and mollusks, are predominant grain types. As indicated by large sand waves, sediments are carried to the bank edge by dominant offbank-
oriented currents that are generated by storm winds. At the bank edge, a wedge of sand, up to 2 1 m thick, may form (Hine and Neumann , 1977; Palmer, 1 979). Much of the shallow-
Bank Margin
475
F ig u re 1 7-Aerial photog raph of tid a l
deltas of carbonate sand formed in
channe l s between isla nds near Cu rlew
Cay, Little Bahama Bank.
water sand is transported off the marginal escarpment , ultimately to be deposited on the deep flanks of the carbonate platforms. Along windward margins, par ticularly those fronting large islands such as Grand Bahama Island on Lit tle Bahama Bank, marine sand belts can form at the bank margin in water depths of 30 to 50 m. There, skeletal sands (mostly algal) are transported over the bank edge by currents in itiated by storms. Deeper water reefs along the margin may act as dams, behind which the sands accumulate as thick as 7 m (Fig. 1 5). Presumably during storms, water piled up against the large islands sets up seaward return flows at the bottom. The com bination of a sloping sea bottom and the seaward-directed currents can transport skeletal sands to the bank edge and over the escarpment. Marine sand belts can form in
association with longshore transport in the nearshore zones of large islands. The mixed carbonate terrigenous sand body Escollo de Arenas off Viegras Island east of Puerto Rico is a good example (Rodriguez, 1 979). This sand body ex tends 6 km off the west end of the island; maximum width is 1 km and maximum thickness is 1 5 m (Fig. 1 6). The top of the shoal lies in 4 m of water and is dominated by a series of widely-spaced, symmetrical sand waves that respond to nearly equal ebb and flood tidal currents . During a former low stand of sea level, siliciclastic sands were brought to the west end of the island by longshore currents . As sea level flooded the platform, these sands were left on the shelf in a manner similar to the cape associated shoals or cape-retreat massifs of the United States east coast shelf (Swift et al, 1 972). Tidal
currents reworked these sands form ing a large sand-wave field, and car bonates (primarily algae and mollusks) were introduced from the shelf lagoons. A carbonate bank, which occurs along the northeast shore of Shark Bay, Australia (Davies, 1970), might also be considered a sand belt. This sediment body, 7 to 1 2 m wide and about 1 00 km long, consists of a con tinuous intertidal sand sheet parallel ing the shoreline and cut by numerous tidal channels with associated levees and fans leading off shore. The bank, composed largely of quartz, algal, foraminiferal, ooid, and mollusk sands, contain'S lesser amounts of muddier sediments.
476
Bank Margin
Fig u re 1 8-Aeri a l photograph of tidal deltas in the Exuma Islands. Ooid sands a re forming on many of the deltas. Deep waters of Exuma Sound are to the bot tom of the photog raph.
Figure 1 9-Geologic map of Florida shelf by Enos (1 977). The area mapped
as bare sand (white and blue d iagona l stripes) corresponds with White Ban k . Note that active reefs and dead
"rubble" reefs are shown in red, and
patch
reefs are shown in black.
lE G E N D � �
LAND ROCK, � E ST R I C T E O l b MU D SAN D
{GRASS {G�ASS
2o BA R e 2 b Jo
BAR E 3 b
-
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U N O I F FF.R E NTIATED
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R OC K , OPE N lo REEF 5 RU B B L E &
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Bank Margin
RECENT S E DI M E NT 0
KEY LARGO
D
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MIL E
HAWK
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PLE ISTOCE N E ROCIC 0
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t
WH ITE
BAN K
THE ELBOW
0
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5
u
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10
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15
Fig u re 20-C ross s ect io n across the Florida shelf by Enos ( 1 977) show ing
sediment thickness (i n yellow) and sedimentary structures as revealed by box coring. The box core from White Bank (also highlig hted in yellow) in dicates that the clean skel eta l sands are c ross-bedded . The structures have been partia lly obl iterated by bu rrowing and by g rass roots toward the base of the core
on the right (f rom Paul Enos, 1 977).
TIDAL DELTAS
Lobate or fan-shaped flood and ebb tidal deltas formed of ooid or skeletal sand may occur opposite inter-island gaps or rocky tidal inlets. Small islands along carbonate bank margins are commonly lithified. As a result, the passes or gaps between these islands do not change position substantially with time. The associated sand bodies therefore, are not exact analogues of the tidal inlets found along siliciclastic barrier island coasts. The shallowest portions of tidal
deltas respond to daily tidal flows. Bed forms (ripples, mega-ripples, and sand waves) consisting of ooid sands are common in these areas . Deeper portions may consist of skeletal sands, partially stabilized by sea grasses and algae. The bedform distribution, including symmetrical sand waves, indicates that significant migration of the tidal delta does not occur during normal tidal events. Migration probably occurs during storm events, when water piled up against the islands forms a steep hydraulic gradient. As a result, strong currents may be generated in the channel between the islands. Between storms the tidal delta is essentially stabilized, except for the shallowest portions over which water flow is accelerated. Large ebb-dominated tidal deltas, illustrated by Purser and Evans ( 1 973) from the Trucial Coast area of the Persian Gulf, extend 1 to 2 km off-
shore and are 5 to 6 km wide. They are wave-modified as shown by the numerous subtidal bars superimposed on them. Ooid sands forming on the deltas are transported to the adjacent islands. Many tidal deltas are quite small, sometimes only a fraction of a square kilometer, such as those il lustrated by Jindrich ( 1 969) and Basan ( 1 973) from the southern Florida Keys. Considering the numerous small islands and island chains on the Bahama Banks, the number of tidal deltas is certainly large . The best ex amples are found in the Carter and Strangers Cays area of northeastern Little Bahama Bank (Fig . 1 7) and in the Exuma Islands of Great Bahap1a Bank (Fig. 1 8).
Bank Margin
478
Figure 2 1 -Aerial p hot ograph of the skeletal sand belt that lies immediately landward of the Bel i ze ba rrier reef (darker band along left margin of sand belt). The reef and back-reef sands, about 1 00 m wide, fo rm the major topographic feature between t h e Atlantic Ocean and the Belize shelf-lagoon.
Figure 22 A eri a l photograph of a car bonate sand shoal and island, located on a topographic high and surrounded by deep water, in the Persian Gulf east of Qata r peninsula. The island is about 1 km in leng t h The topographic h ig h is probably the su rface e xp r e ss ion of a salt d om e -
.
.
BACK-REEF SANDS
Sand bodies frequently occur land ward of reefs , because reefs often produce an abundance of sand. White Bank, a shoal area about 5 km east of Key Largo, Florida, is such a sand body (Enos, 1 977). Algal, mollusk, and coral sands form a belt that is 1 to 2 km wide and 40 km long (Fig. 19). Although large portions of the
belt are stabilized by sea grass, white rippled sands are common (Fig. 20). Both active and stable bottoms occur at similar water depths, 1 to 3 m, suggesting that scattered large sand waves must be due to localized hydrologic conditions. The sand body is asymmetrical in profile; the steeper lagoon side suggests bankward transport of sand. Large elongate algal sand lobes
have also built into the back-reef lagoon of Bermuda (Garrett and Scoffin, 1 977; Garrett and Hine, 1 979) and project as much as 4 km into the lagoon. They are up to I km wide and 15 m thick. Small reefs, which contribute sand grains other than algae, are common on top of the lobes. Seismic data indicate that the positions of the lobes are controlled in part by underlying topography of Pleistocene bedrock highs. A much more extensive sand body occurs landward of the Belize barrier reef (Fig . 21). Although only 100 to 200 m wide, it is continuous and uniform with a total length of about 1 60 km and unbroken lengths of about 30 km . This sand belt probably owes its exceptional uniformity to the well-developed barrier reef that lies immediately seaward of it. The sedi ment surface is very flat. The sands abut the reef in 1 . 5 m of water and drop off into a shelf lagoon toward the west. The slope into the lagoon may be as steep as 35°, and the sands appear to be burying lagoonal patch reefs in some areas . Islands occur within the sand belt, usually near breaks in the reef.
Bank Margin 479
Figure 2 3-Ae rial photograph of large ooid and skeletal sand waves i n Mexico located between Isla Mujeres on the right and the Yu catan mai nland on the left. I s la Mujeres is approximately 8.5 km long. I ndividual sand waves are as high as 5 m .
Figure 24-Landsat i m a g e of the Joulters ooid shoal, lying north of Andros ' I sland (shown in red) on Great Bahama Bank. Deep-water Tongue of the Ocean is to the upper right. The ooid shoal is a vast sand flat, 400 sq k m , t h at is penetrated partially by tidal channels and fring ed on the eastern border by mobile sands. Holocene islands are scattered along the mobile fringe and throughout the sand flat. Contrast the size and morphology of the Joulters shoal with shoals pictured in Fig u res 6 and 1 3.
480
Bank Margin
Figure 25-Photomicrograph of filamen tous, calcifying algae that are acting as the primary binding and cementing agent in ooid sands from the Schooner Cays area, Great Bahama Bank. Magnesi u m -calcite is artificially stained reddish-brown; a r ag on i te remains un· sta ined. Width of the photograph represents 3 mm.
MISCELLANEOUS SAND BODIES
Within the interior of carbonate banks and platforms, active sand shoals, commonly oolite, may be situated on or near topographic highs. These sand bodies respond primarily to wind-generated currents; tidal flow is substantially diminished by the time it travels 20 to 50 km across the platform. Mackie shoal on northern Great Bahama Bank in the Bahamas is an excellent example. Seismic data indicate that a rock ridge may have localized this 1 .5 km wide, 7 m thick, 30 km long linear sand shoal (Hine et al, 1981). Reefs and sand shoals form on structural rises in the Gulf of Mexico and the Persian Gulf. Many of these accumulations overlie structures pro duced by salt diapirs, an example be ing the Flower Gardens reef about 200 km south-southeast of Galveston, Texas (Edwards, 1971). Purser ( 1 973) has classified similar bathymetric highs in the Persian Gulf as being outer (in the basin center), in termediate (Fig. 22), or inner (coastal). The sediments on these
highs vary depending on their loca tion. In the basin center, they are characterized by open-marine, foraminiferal or coral/algal sands. On highs located near the coast, sediments are composed of pelletoidal sands, carbonate muds with restricted faunas, and algal stromatolites and evaporites . Where the highs exceed 5 km in width, double sediment spits or "bull horns" form curving elongate barrier sand ridges. These tails pro duce sheltered lagoonal conditions over the center of the structure. Islands develop on many of these shoals, and the exposed sediments are subjected to diagenesis while in con tact with fresh waters. Unexposed sands which occur on highs in more than 10 m of water are extensively lithified by marine cements. The geometries (lobate or linear) described in the preceding sections are relatively simple and represent end members in a continuum of shapes of possible carbonate sand deposits. Un doubtedly, many carbonate sand ac cumulations do not fall into one of these described categories because they are unique, their geometry con-
trolled by the peculiarities of geographic location. An example of such a shelf-edge sand accumulation is one located off the Yucatan Penin sula of Mexico (Harms et al , 1 978; Fig. 23). Other sand bodies may be combinations of two or more of the end-member forms, or they may in clude large areas of inactive, stabiliz ed sand. SUMMARY OF FACTORS INFLUENCING SAND BODIES
Because bank-margins are the primary zones of energy absorption and because the duration and magnitude of physical energy can fluctuate as a result of changes in bank orientation and topography, im portant variations in sand-body size, geometry, and sediment type should be expected. The previous discussion has pointed out these variations. The development of sand bodies re quires, in the simplest view, a source of sand-size sediment and a mechanism to exclude or remove sedi ment smaller than sand-sized material . On shallow carbonate
Bank Marg in
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.
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banks, platforms, or shelves, the sand source is almost always nearby (autochthonous). The sand may be of biological or physicochemical origin . A change i n the shelf slope may pro vide a focus for factors that promote carbonate sand production including wave impingement on the bottom, upwelling, strong tidal currents, and enhanced biotic activity. If the slope break faces open water that is more than 10 m deep, there exists enough
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wave energy commonly to remove particles finer than sand size. Given these two generalities - a sediment source and a mechanism for removing the fines - then sand bodies will develop near the shelf edge. Several other factors contribute to form sand bodies with differing attributes. The predominant influences on sand production and the development of carbonate sand bodies are : ( l ) antecedent topography; (2) physical
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.
Fig ure 26--Seisrnic profil e and inter· pretation along a leeward bank margin the Bahamas. Numerous re·flectors wllhln tf'le thiCk sand accumulation are the res�:�lt of hardgrounds, thin laterally cor.t inuous lith!fied layers i n the other wise uncemented sands. The hard· grounds are formed by submarine t:f:!rTienllJiion of c�:�rbonate sands.
in
482
Bank Margin
F i g u re
27-Photomicrograph of a th in for m i ng a cemented ooid crust, from the Joulters ooid shoal on Great Bahama Bank. This habit of a ragonite cement is termed fibrous or needle. Width of t he photog raph represents 1 mm. fringe of a ragonite cement
Fig u re 28-Underwater p h ot og r a ph showing friable clasts and boulders from the Schooner Cays area of Great Bahama Bank. The clasts, cemented with fibrous aragonite marine cement, are common on the bottom ti dal chan nels that lie between active sand shoals (���- Fig; 7);
processes - tidal, wind, and wave generated currents; (3) sea-level history; and (4) diagenesis. All the factors provide varying feedback ef fects on one another, resulting in a strongly integrated system of en vironmental controls. No one factor can be examined without considering the influence of the others . Antecedent Topography The positioning of a sand body along a bank, platform, or shelf
margin may be the result of underly ing topography that localized agitated bottom conditions. The antecedent topography may be in the form of submarine rock ridges, gaps between islands, reentrants within the bank margin, gradient and elevation of the bank-margin zone, and sheltering ef fects of other sand bodies. For exam ple, the Cat Cay ooid shoal on Great Bahama Bank (Fig. 1 1 ) has formed on submarine rock ridges that con nect small Pleistocene islands along
that margin . The rock ridges, after being flooded during sea-level rise, locally focused tidal flows on and off the bank into small areas. This in turn stimulated ooid production and the sand body began to develop. Another example is the tidal deltas produced at the mouths of channels between Pleistocene islands in the lower Florida Keys (Jindrich, 1 969). The gaps or passes between the islands are obvious examples of antecedent topography controlling the
Bank Margin
F i g u re 29-Photomicrograph
483
of
cemented ooid c rust with a l l of t h e p ri ma ry inte rp articiA poros ity f i l led with
fibrous a rago ni te cement. Note the po lygo n a l cement boundaries s u r round
ing each ooid. S ample from the Yellow Bank a rea of Great Bahama Bank. Width of photograph represents 2 . 5 m m .
�igu r e 30-P�loturn ic rog rap�l of
cemented oolite f ro m the Schooner Cays, Great Bahama Bank. M icritic magnesium-calcite cem ent , artifi cially
stained reddish-brown, fills most of the porosity. Note that it postdates an earl ier g en e ration of fibrous a r a g o n ite cement that i nit ia l ly ceme nted the rock. Width of the photograph represents 3 m m .
placement and development of the tidal deltas. The location, size, or orientation of some sand shoals shows no obvious correlation with underlying preex isting topography, but important genetic relationships may still exist. Seismic profiling indicates that Lily Bank on Little Bahama Bank was not controlled by underlying rock struc ture. However, antecedent
topography in the form of two broad topographic lows or reentrants within the outer bank-margin has played a key role in shoal development (Fig . 1 3). These high-energy windows along the windward margin provided the currents necessary to create an agitated bottom at the shoal location. Whether the relation between bedrock topography and initial deposition is obvious or difficult to
document , the effects may only be fleeting as sediment accumulates and buries the bedrock inrregularities. Ap parently with the accumulation of sufficient sand , the sand body assumes its own characteristics, so the resultant size and orientation of Holocene examples are to a large degree products of the hydrodynamic regime and related depositional buildup.
484
Bank Margin
Figure 31 -Photomicrog raph of oolite from Jou lters Cay, Great Bahama Bank, cemented with calcite which has p recipitated from fresh-waters held by capi l l a rity above the water table (vadose zo ne) . Note the tendency for the cement to occu r at g rain contacts and to have c u rved surfaces that are concave toward the pore interiors. Width of photograph represents 1 .5 mm. .
Fig u re 32-Photomicrograph of oolite Cay, Great Bahama Bank, cemented below the f resh-water table (phreatic zone) with calcite. Cement oc c u rs as rims of c rysta ls th a t a re uniform ly distributed around each ooid. Note that the rims are not of even thickness (non- isopachous}. Width of photograph represents 3 mm. f rom Jou lters
Physical Processes
The orientation of a bank-margin with respect to the dominant winds, waves, tides, and storms imparts a significant effect on sand body type. The duration, magnitude, and direc tion of water movement shape the sediment body and produce physical characteristics which may be preserv ed in the rock record . Bank margins can be classified as : ( 1 ) windward, (2) leeward, or (3) tide-
dominated. Back-reef sheets, belts, and lobes of skeletal sand form along open windward margins, where sand transport is generally toward the bank. Where small islands exist, oolitic tidal deltas occur opposite the inter-island gaps. Commonly the flood tidal delta is much larger as a result of storm-generated currents. If reentrants occur along the shallow margin, tidal and storm-generated flows can generate wide belts of tidal
bars or marine sands. Along wind ward margins dominated by large islands, skeletal sands generated within the fore-reef environment may be carried seaward to the marginal escarpment . There they accumulate behind rocky barriers or are carried further seaward into deep water. Leeward, open margins are dominated by offbank sand transport and wide belts or sheets of non skeletal sands form at the bank edge.
Bank Margin
485
Figure 33-Su rface photograph at low t i d e of extensively rippled bottom of Joulters ooid shoal on Great Bahama Bank.
F ig u re :14-Surface photograph showing deta il of ripples. A pattern of nearly parallel ripples has been modified by
currents during the fal l i ng tide. The fecal mound i n the center is nearly 3 em h i g h .
At the ends of embayments, where tidal currents are accelerated, large tidal-bar belts characteristically form . Sea-Level Changes Sea-level changes can significantly control the location and development of carbonate sand bodies because the water depth over the bank controls the level of physical energy imported to an area. Rapid changes in sea level may isolate or even drown sand bodies on different parts of the shelf,
while slow changes may produce widespread sand sheets. For example, Pleistocene oolites deposited about 20 thousand years ago occur at the shelf edge of the eastern Florida peninsula, now about 63 m below sea level . Dur ing a sea level high stand, about 1 30 thousand years ago, oolitic deposits of the Miami Oolite were formed 1 6 t o 24 k m bankward from the shelf edge. These distinctly separate sand bodies, deposited during a very brief span of geologic time, owe their
origins to a rapidly fluctuating sea level . In contrast, slow migration of a Devonian crinoidal sand shoal during a relative sea level change produced the widespread encrinites of the Becraft Limestone in New York State (Laporte, 1 969). The geologic histories of both Lily Bank on Little Bahamas Bank (Hine, 1 977) and the Joulters ooid shoal on Great Bahama Bank (Harris, 1 979) are tied very closely to the Holocene rise in sea level. In both cases, a wide
Bank Margin
486
Fig u re 35 - P i astic- i m p reg nated surface of a box core, approximately 1 5 em
ac ross, illu strating the small-scale, r i p p le c ross-bed d i n g that c h a racterizes active
port ions of sand shoals.
.
.
.
Fig u r e 36-Photomicrograph of clean
sands from a core taken on the active
sand shoal shown in F ig u re 33. The ex·
tremely well -s o r ted , m e d iu m sand-size ooids a re in l a m i n a t io ns t h at dip slig htly to the r i g h t . Thin beds of loosely-packed, coarse g ra i n s (algae , fora m i n ifera, and aggregate g rai n s) also dip to the r ight The i ncl i ned laminae a re defined clea rly where there a r e va riations i n g ra i n s i z e ; t h ey a r e more subt l e i n a w e l l - so rt e d o o i d san d (for instance in the bottom one - t h i rd of the photog raph). The scale
act1ve sand sheet cons1stmg both of parallel-to-flow bed forms and large sand waves was developed soon after sea level had flooded the bank top with a few meters of water. Parts of at these sand bodies became relict and stabilized by vegetation as they were not able to build vertically as quickly as the rising sea. Portions did remain active, though. Lily Bank today is a mobile sand belt that has been reduc ed considerably in width; in the Joulters example, the active shoal has shed considerable sand bankward to form a large sand flat and has also built above sea level along much of its length (Fig . 24). Diagenesis Several diagenetic processes may greatly alter the character of car bonate sands while they are still essentially within the depositional en vironment . These processes , occurring before or shortly after final deposi tion, while the sands are at or within a meter of the original depositional surface, fall into categories of grain alteration and cementation . Grain alteration begins essentially as the grains are produced and is a
the bottom is in m i l l i meters.
continuing but sporadic process while sands accumulate. Alteration of grains by repeated microboring and infilling of microbores can completely destroy original grain fabrics (Purdy, 1 968; Harris et al, 1 979). Often such alteration is confined to the grain edge and produces micrite envelopes (Bathurst, 1 966). Micrite rims can also form by the addition of fine grained material to grain surfaces (Kobluk, 1 977). These material addi tions are usually aragonite or Mg calcite. The added material may bridge the space between grains, resulting in grain-to-grain cementa tion (Dravis, 1 979; Fig. 25). Unlike their clastic counterparts, carbonate bank-margin sand bodies may be subjected to syndepositional submarine cementation. These cements result in the formation of clasts, hardgrounds, and "tepee"
structures in shallow marine water (Shinn, 1969). Although the details of inorganic marine cementation in car bonates are unclear, the general pro cesses which enhance cementation are: ( 1 ) good water circulation (shallow-marine waters overlying car bonate sediments are typically super saturated with respect to the major carbonate minerals); (2) high sedi ment permeability; (3) slow sedimen tation rates ; and (4) hypersalinity. Marine cementation may exert a significant influence on the subse quent development of the sand body. Submarine hardgrounds that com monly develop on the flanks of sand bars and lithified layers that form on beaches combat erosion and play a role in maintenance of steep slopes (Fig. 26). These hardened bottoms also aid in the establishment of a ben thic community that could not survive on an unstable bottom. Bliefnick (1 980) has documented the growth of sponge and bryozoan buildups on marine-cemented sea bot toms of the Joulters ooid shoal on Great Bahama Bank. To the north of the shoal, the open bank margin has changed through time from an en vironment of active tidal bars to one of an essentially stabilized bottom with the widespread occurrence of lithified layers and patch reefs. A similar scenario may explain the
Bank Margin
. �.._ �
487
. �.... �;; .::,/���
F igure 37-Su rface ph otog raph at ex tremely low tide exposing spi l l -over lobes
on the backside of an active ooid shoal
at Joulter Cays on Great Bahama Bank. Clea n , c rossbedded ooid sands are cove ring bioturbated, muddy ooid sands that have been partially stab i l ized by marine g rasses and algal mat.
Figure 38-Aerial photograph of sand
flat lying west of the Joulters Cays on Great B a ha m a Bank. The mounds, pro duced primarily by b u rrow ing s�nirnp and
c rabs, are spaced less than 1 m apart. Photograph taken f r o m about 70 rn a ltitud e .
association of ooid shoals and certain organisms reported by various authors such as : stromatolitic bioherms associated with late Precam brian oolite of the Canadian Ar chipelago (Young and Long, 1 977); archaeocyathid-Rena/cis bioherms in terbedded with Lower Cambrian oolites of the Poleta Formation of western Nevada (Rowland, 1 978); and Epiphyton boundstones with Cam brian and Ordovician oolite shoals of
the Central Appalachians (Pfeil and Read, 1 980). Physicochemical marine cements commonly take the form of fibrous aragonite or micritic Mg-calcite. Nar row fringes of aragonite-needle cements are sufficient to lithify car bonate sand into friable clasts and hardgrounds (Figs . 27 , 28). Fibrous, isopachous cements may form polygonal cement sutures around grains (Shinn, 1 975; Fig . 29) resulting
from competitive cement growth into pore spaces. Micritic Mg-calcite ce ment can occur as the sole cement in a rock or it can be associated with aragonite cement, often as a second stage cement after the aragonite (Fig. 30). Mg-calcite cement may also oc cur in a fibrous or bladed habit, but this form does not appear to be com mon in Holocene sands. Continued cement growth can greatly reduce porosity and permeability and destroy
488
Bank Margin
Figure 39-Surface photograph at low tide showing details of bu rrow mounds. The white mounds of pelleted sand are sur rounded by grass- and algal- covered bottom . The mounds have been reworked by the tida l currents, g iv i ng them a flattened appea rance.
Fig u re 40-Photomicrograph of muddy
sands from a core taken on the sand flat shown in Figure 39. Ooid sands are
bioturbated and mixed with mud and
other carbonate sand g rains. The m u d , appearing a darker gray, h a s a n i r regular distribution within the sediment due to the burrowing. Width represents about 30 m m . left side represents bottom
ol photomic rograph.
the reservoir property of the sands very early in the history of the rocks. Fresh-water cements may also significantly affect the development of sand shoals. Islands and dune ridges are rapidly cemented in Baha mian ooid shoals to form erosion resistant features. The islands initiate growth of beaches on their seaward sides. Also, they may significantly change current patterns and deposi tional environments by forming rigid barriers and creating energy shadows
in their lee. The islands also serve as catchment areas for fresh waters, and mixing zones form around the islands where the fresh water and marine water intermix. Although investigations are conti nuing, it is believed that calcite cementation, dissolution, and possibly dolomitization characterize the diagenesis associated with island for mation. Islands act as traps for ground water that differs significantly in chemical composition from the
marine waters surrounding the island. Often, as in the case of Bermuda (Plummer et al, 1 976), clearly recognizable hydrologic zones develop beneath an island. These include: ( I ) the vadose zone (that portion o f the island above the standing water table): (2) the phreatic zone (those sediments saturated with ground water below the water table); and (3) a mixing zone (the zone of mixed ground water and sea water). The waters in these zones and the
Bank Margin
·;.-···
..,- 1: .i '.:.
489
.. .
: �
�-- 5 0CMI--SAND VEI>E!R
20M
·
1-lGt..OCEI\E H:EFS
[.::_]
g
HOLOCENE SANJ Fl l'"ISTOCENE ROCK -- ------'
F ig u re 4 1 Se i sr:1 i c profile trcm the leewarc' marQ in of Li1t!e Bahama Bank. The i!lterpretation of the profile and 1he mooal tor its davelopmaot t h rough time sl1oV'; P1GU an early Holoce n e corc�1 reei was nuded b�i la te r H olo cene sands. The sands formed on the platform only after it was substantially tlooeled durin9 riSi"lg sea level. A similar scenario hoB been p roposed for portions of the platform surround1ng To:1gu e of the Ocean on Great Bahama Ba(li< (Palmer. 1 979).
7,000 YRS. B.P.
RISING SEA LEVEL
10,600 YRS. B.P. REEF
geometry of the zones are controlled by sediment properties and hydrologic conditions which are beyond the scope of the present paper. However, illustration of some of the early fresh water diagenetic products will serve as analogues to some carbonate reservoir
rocks. Carbonate sands within fresh ground water commonly develop meniscus cements above the water table (Dunham , 1 97 1 ; Fig. 3 1 ) . These cements form where water is held by capillary forces between sand grains.
Below the water table, cement crystals grow freely in pore waters between sand grains (Fig. 32). These cements result from the dissolution of aragonite grains and the reprecipita tion of calcite as cement. The process results, in part , from the greater
490
Bank Margin
.'f ....... l.l. '
STRAI rs OF FLORIDA {WEST) -
��1[1--��� · · · · · · ·· · · · · · · ·· ·--- -- - · ·. -- - -- - - · - · - - ---- - ·------ - - · -- -�
- -
:
- ._
... -..
.....
EXPLANATION
I b\U HOLOCENE SAND • ROCK UNIT I
Figure 42-SP.;smic
profile
ROCK UNIT 2
� ROCK lJ\IIT 3 �PRE-ROCK UNIT 3
and inter
pretation from l eeward b a nk marg in of Little Bahar1a Bank. The HolocP.ne ac
c retion overlying Pleistocene limestone "basement" is a complex package of bank margin sands and reefs, with both u nlith ified a11d lithitied carbonate sands.
solubility of aragonite than calcite in fresh water (Land, 1967). Because secondary porosity is created as aragonite dissolves and as cement precipitates in pores, little overall porosity change takes place in the sediment. However, a significant permeability decrease results from this early cementation (Halley and Harris, 1 979). Mixing zones may localize dolomitization (Land, 1 973) or calcite dissolution and precipitation (Run nels, 1 969) . The mixing may take place early or late in the history of the sand body (Hanshaw et al, 1 97 1 ) . O f course, the mixing zone i s not the
only possible mechanism for dolomitization. It is also possible that hypersaline ground waters dolomitize sediment while sinking through a san dy body (Adams and Rhodes, 1 %0), but this process has not been well documented in the Holocene. Hsu and Schneider (1973) have shown that the dolomitization found along the southern shores of the Persian Gulf occurs when sea water is drawn land ward due to extremely high evapora tion rates on the tidal flats. PRESERVATION OF MODERN SAND BODIES
The geologic record of modern sand bodies provides us with sedimen tary criteria that form the basis for the environmental reconstruction of outcrop and subsurface core data. Recent studies in the Bahamas by Buchanan (1970), Hine (1977), Harris
(1979), and Palmer (1979) have added greatly to our understanding of the facies, sedimentary structures, and geometry of oolitic deposits and associated sediments. Surface Sediments Early studies of surface sediments documented the variations of organisms, surface structures, grain types, grain size, and amount of mud that typify bank-margins dominated by sand shoals. The variations are complex and somewhat variable bet ween different localities, but general trends can be recognized. For exam ple, the crest of a shoal itself is usual ly devoid of organisms and biologically produced structures, as physical sedimentary structures predominate in the clean, well-sorted ooid sands (Fig s 33 to 36). Bioturba tion and burrows are more common seaward and bankward of the shoal, .
Bank Margin
491
SECTIONAL V I EW
��L
0 !5 �ILOIIETERS
FACIES
D � � r;:::'l �
-
SKELETAL GRAINSTONE
O O I D G R AI NSTONE O O ID PAC K S T O N E F I N E - PE L O I D P A C K S TO N E P E L L E T W AC K ESTONE
L I THOC LAST PACKSTONE
where ooid sands are mixed with other grain types and mud (Figs. 37 to 40) . Subsurface Facies Relations Seismic profiling , along with sedi ment coring and probing, have shown that modern sand bodies in the Bahamas are less than 20 m thick and commonly less than 10 m . The sand bodies have built either landward or seaward through time, depending on the predominant direction of sedi ment transport. The complexity of platform margins has been graphical ly shown by Hine and Neumann
( 1 977), and Palmer (1 979), who presented seismic and core data along a leeward margin showing early Holocene coral reefs that have been subsequently buried by thick ac cumulations of carbonate sands dur ing subsequent offbank transport later in the Holocene (Fig. 4 1 ) . This change through time is largely a result of the flooding of a greater portion of the carbonate platform during a rising sea level. Continued fluctuation of sea level might result in several ac cretions to the margin (both vertical and lateral growth; Fig. 42). The mosaic of facies that can be
F ig u re 43-Cross section t h r ough the ooid shoal !hal is pictured in Figure 24 (Ha r ris, 1 979j. Beg i nning on tt">e rig ht, t h e section extends f rom the bank· margin for about 35 km into the bank· i nterior. Tho actua! bu ildup of the shoal above the surrounding sea bottom is d u e to the accumulation of ooid sands in several facies. The faci(;!s r e l at,r.m shios refect environmental changes throug h time (from P. M . Harris, 1 979).
recognized in studies of surface sediments forms an even more com plicated pattern when viewed ir. three dimensions; the complexities arise from sediment variation that is the result of topographic irregularity, localized depocenters, and, most im portantly, changing depositional pat terns through time. Bahamian ooid sands originate in several settings - primarily in lobe-shaped or elongate submarine bars, but also in tidal deltas localized in passes bet ween islands. The sands may also ac cumulate in a variety of suben vironments associated with these sites
492
Bank Margin
Fig ure
44-Composite
stratig raphic col·
umn showing the lithologies that c om
pose the shal lowing-upward depositional sequence that has been recognized in
Pleistocene li m e sto ne s of the Bahamas ( P ierson , 1 980). The environmental inter· pretations of each lithology are based on studies of their Holocene counte rparts.
Fig u r e 45 -0 u tcrop photograph ot Pleistocene Miami Oolite exposed in F l o rida. The c ross-bedded ooid sand was deposited as a marine bar like that shown in Figure 3 3 . Note the excellent preservation of the cross-bedding as a
result of selective cementation and subsequent weathe r i n g .
of formation. An ooid shoal is com prised of a succession of these en vironments produced through time during changing sea level. Changes in sedimentation through time have best been documented by coring on Great Bahama Bank north of Andros Islands. Buchanan (1 970) found that deposition in the Frazers Hog Cay area was initially influenced by an elevated rim of Pleistocene limestone along the seaward edge of the platform which effectively restricted circulation during early flooding of the platform . Lime muds accumulated in the lagoon behind the
ridge. As the ridge was flooded by the rising sea, lime sands formed . Ooid shoals and skeletal sands occur at the platform margin while pellet, organic aggregate, and grapestone sands are found bankward. The scenario is similar for the Joulters ooid shoal located nearby. The Joulters shoal today is not a tidal bar or marine sand belt, but rather a vast sand flat fringed on the ocean-facing borders by mobile sands where ooid formation occurs. Harris (1 979) has documented the geologic history of the shoal, showing one possible evolutionary path of the bar
and channel pattern seen elsewhere. The Joulters example is important because it shows that the shoal generating physiography can be erased - the bar-and-channel topography has been extinguished and filled in, and clean ooid sands have been mixed with other sediments by burrowing to form a sand flat. The relief of the Joulters ooid shoal over the surrounding sea floor is the result of the accumulation of ooid sands in several facies. A cross section (Fig. 43) reveals two basic parts to the shoal : ( 1 ) a narrow fringe of ooid grainstone, and (2) a more
Bank Margin
493
F i g u re 46-Photomicrog r aph of Pleis t oc e n e oolite from the outcrop
s hown in Figure 45. The oo i ds are c e m e n ted with rims of calcite crystals i nt e rpr e t e d to have been cemented in a fresh-water p h reatic environ m e n t . The
o o ids have undergone si g n i f ican t only r t; r tltll:lrrt s of ltle
d issolu tion, so
o r i g in a l aragonite remain. Width of photog r ap h
widespread sequence comprised of lithoclast packstones and/or pellet wackestone at the base, peloidal packstone in the middle, and ooid packstone at the top. The ooid packstone forms a wedge that thickens in a seaward direction, while the peloidal packstone is the thicker part of an intraplatform sheet that thins in a seaward direction. Shallowing-upward Sequence The sequence of deposits recogniz ed in cores from the Joulters ooid shoal and other Holocene examples , as well as in cores of Pleistocene limestones in the Bahamas, is one in which each succeeding unit is deposited in progressively shallower water. This is largely because shallow water sites provide optimum condi tions for the physicochemical and biological production of carbonates, so platform carbonate sediments usually accumulate at greater rates than the rate of relative subsidence and repeatedly build up to sea level and above. This shallowing-upward sequence, the facies of which can be analyzed and related to an existing depositional environment in the Holocene, may be repeated numerous times in a thick succession of ancient platform carbonates and provides a valuable clue to their interpretation (Fig. 44).
Figure 47- 0utcrop photograph of
Pleistocene o o l i te from New Providence I sland, Bahamas. The sedimentary struc
t u res i n d i cat e a ch a n g e of depos itional e nvironment toward the top of the se
q u ence. Complex c ross-bedding of a n a ct ive m a ri n e bar is dramati ca l l y
d isplayed throughout most of t h e sec· tion; but towa r d the top. flat-lying
l a m i n a e record the localized buildup to beac h condit ions.
r
ep resenls 1 .5 rnm.
The sequence of facies documented in the Holocene of the Bahamas is formed on a weathered surface of Pleistocene limestone. The surface is typically pitted and coated with a calcareous crust which forms a recognizable regional discontinuity surface that resulted from subaerial exposure during a sea level low . The Holocene shallowing-upward se quence results from depositional buildup during the subsequent flooding of this surface . The initial deposits of the sequence are a transgressive basal lag of coarse coquina or lithoclast packstone. As increasing water depth creates a sub tidal lagoonal environment, pelleted lime mudstone and wackestone are deposited. With improved circulation and more varied biota, the sediments gradually change to bioturbated wackestones , locally packstones, that may be rich in foraminifera and mollusk shells. Initiation of sufficient tidal flow across the platform may cause the localization of ooid forma tion on agitated bottoms. Ooid sands may be mixed with the lagoonal sediments that surround a shoal. If a shoal overlies the lagoonal deposits due to a transgression or prograda tion, the vertical sequence grades into laminated intertidal ooid grainstones. Burrowed, shallow-subtidal ooid sands may be present as well, the
494
Bank Margin
F ig u re 48-0utcrop photograph s h owing closer view of cross-bedde
deposits of Fig u re 47. N o t e the c u t-and f i l l structure of va r i o u s cross-bed sets.
The vertical holes a re preserved burrows of m a rine worms and anemones. The scale ba r is in centimeters .
Figure 49-0utcrop photograph showi ng closer view of planar-bedded beach dep osi ts of Fig u re 4 7 . Note that there are gradual pinch-outs of the bed sets i n
both di rections. A prom i nent set of cl imbing ripples is present immediately above the scale. The scale bar is i n centimeters.
result of sand flat deposition. When the supply of sediment is large, the sequence may be capped by bedded supratidal ooid grainstone of beach ridge or dune deposition . In the Bahamian model of deposi tion during the flooding of a car bonate platform, the sediment surface builds up to and exceeds sea level. The upward change of facies, form ing in depositional environments that were superimposed through time, is a
shallowing-upward sequence. Clues to interpreting the water depth of such deposits lie in the grain types, sedimentary structures, depositional texture (percentage of mud), and diagenesis. ANCIENT SAND BODIES
Sand body geometry and size, paleogeographic location and orienta tion, and internal structure are key
criteria to use in the interpretation of carbonate sand bodies. Usually, they cannot be determined from subsur face studies until late in the explora tion of an area. As a result, the preliminary environmental interpreta tions are often based on grain type only. For instance, oolites may become ooid shoals, encrinites become crinoid banks, and caprinid sands become rudist thickets. Such in terpretations are probably best withheld until more data are ac cumulated, but the interpretations may serve to inform initial explora tion. Subsurface sand bodies, when thoroughly explored, may reveal the geometries of individual sand bars or back reef sands. For example, bar morphology has been documented for several sand bodies - the Mississip pian McClosky sands, an informal unit, of the Illinois Basin by Carr ( 1 973) and the Jurassic Smackover Formation of the Gulf Coast by Er win and others ( 1 979). Pleistocene Outcropping carbonates provide the most readily accessible and easily identifiable examples of ancient car bonate sand bodies . Pleistocene out crops often retain much of their geometry left relict during exposure associated with sea level fall. In the
Bank Margin
495
Figure 50-0utcrop photograph showing another view of the cross-bedded marine deposits of Figure 4 7. The layer of reworked limestone blocks (intraclasts of
marine-cemented ooid sands) is typical of lithification that occurs on the flanks
of marine bars and on beaches.
Figure 51-Stratigraphic cross section
of the Upper Jurassic Smackover For mation and Buckner Member cf the Haynesville Formation in Arkansas and Louisiana. Note that the Reynolds oolite, a thick accumulation of oil - bea ri n g high energy carbonate sands, changes upd:p to aggregates (grapestones) and car bonate mud. Shales and anhydrites of the Buckner Member act as seals fer Smackover reservoirs.
DATUM-TOP OF BUCKNER FORMADON
SOUTh LAKE ERLING
TAYLOR
OORCHEAT
-MACEDONIA
CALHOGN
FII:..LO
MAGNOLIA
FtELD
5tE:P'H€NS
N0H!H
�
u SMAC>
0 [ill
f
� �
� • m � � ..
MILES
FROM
AKIN &
GRAVES,t969
case of the Pleistocene Miami Oolite of south Florida, the geometry of the marine deposits is quite similar to that of analogous modern sand bodies (Hoffmeister et al, 1 967; Halley et al, 1 977). The Pleistocene of south Florida is
CHILDESTtR
CAMDEN
divisible into five marine units (term ed from oldest to youngest Quater nary I through Quaternary 5), which are separated by regional discontinui ty surfaces (Perkins, 1 977). Paleotopography developed on the upper surface of the Q4 unit created
AREA
SHALE
NONMII.RINE ANHYDRITE ANHYDRITIC
SANDSTONE AND
SHALE
CALCARENITE-CARBONATE MUDSTONE
FACIES
MODIFIED GRAPESTONE GRAPESTONE
FACIES
FACIES
OOLITE FACIES
INTERBEDDED SHilL Y
lO
FACIES
DENSE
LIMES TuNE
a gently-dipping platform on which Q5 sediments were deposited. During deposition of Q5, tidal-bar deposits of the Miami Oolite formed along much of the eastern coast of south Florida. The sediments are ooid grainstones and packstones, locally
496
Bank Margin
Fig ure 52-Photograph of core slab from the Upper Jurassic Smackover For mation in MI. Vernon Field, Columbia County, Arkansas. Porous ooid grainstones have distinct centimeter thick layering. The larger grains scat tered within the oolite are algal balls. Porosity is 12% and permeability is 20 md.
arenaceous and often mixed with pellets. Extensively-burrowed pelletal sands accumulated in sheltered water behind the oolite bars. On outcrop, fresh-water cementation and dissolu tion have greatly enhanced recogni tion of the sedimentary structures in the Miami Oolite (Figs. 45, 46). Large cross bed sets typify the niarine bar deposits. Pleistocene outcrops in the Bahamas broaden our perspective of preserved depositional environments. Marine bars, beaches, and subaerial dunes are exposed on New Pro vidence Island (Garrett, unpub . data) . Ooids occur in ridges or irregular mounds that in outcrop may show a gradation upward from accretion and cross-rippled bar deposits to laminated beach deposits containing displaced beachrock blocks (Figs. 47 to 50). The dunes are composed of marine sands piled up from the beaches by wind and stl'r'll s . They
Figure 53-Photograph of core slab from the U pper Jurassic Smackover For mation in Walker Creek Field, Columbia County, Arkansas. These porous grainstones are a mixture of ooids. algal balls (b o th oncolites and rhodolites) and large pelecypod fragments. Porosity is 8% and permeability is 1 0 md.
are formed of coalescing sand lobes that are asymmetric toward the plat form interior. Sedimentary structures consist of distinctive backset beds with low-angle bedding that change in a bankward direction through convex upward spillovers to steeper foreset beds. The foresets change laterally in to bioturbated sands, which were pro bably located in the lee of the developing dune. Jurassic of the Gulf Coast
Shallow-water carbonates of the Upper Jurassic (Oxfordian) Smackover Formation of the Gulf Coast are one of the best documented ancient examples of hydrocarbon reservoirs in carbonate sand bodies. Imlay ( 1 943), Dickinson ( 1 968), Akin and Graves (1969), and Badon ( 1 974) described the stratigraphy of the Gulf Coast Jurassic and summarized the environments and lithologies of the Smackover. Detailed field studies are
Figure 54-Photograph of core slab showing Upper Jurassic Smackover oolite from Mt. V ern o n Field, Columbia Cou nty , Arkansas. The distinctly cross
laminated ooid grainstone has significant oomoldic poros ity .
presented by Bishop ( 1 968, 1 97 1 ) , Ottmann and others ( 1 973), Chimene ( 1 976), and Becher and Moore (1 976). The Smackover contains promising exploration targets and proven pro duction (Newkirk, 1 97 1 ). Structural traps related to salt movement have long been exploration targets, but now major objectives lie in more sub tle stratigraphic traps. These may show little or no closure and may escape detection on seismic profiles . As a result , understanding the geology of modern carbonate sand bodies can play an important part in the exploration thinking for the Smackover trend. The Smackover deposits in southern Arkansas and northern Louisiana were deposited in en vironments ranging from nearshore to basinal (Fig. 5 1 ). Quartz sands and fine-grained carbonates mark the up dip limits of the Smackover; basinal limestones far downdip contain shales
Bank Margin
497
Fi g ure 55-Density log cross section, Walker Creek Field. Orange areas have
porosity greater than 4% and are con tinued to discrete horizons within the field (from Becher and Moore, 1976).
Figure 56-Inferred distribution of
grainstones and mudstone r esu ltin g in
porosity zonation within the Smackover Formation as illustrated in Figure 55 (from Moore, 1980).
and sands. These basinal deposits ex tend to the north where they are overlain by a belt of shallower-water, high -energy carbonates that extends across southern Arkansas. The shelf deposits, locally termed the Reynolds oolite, are primarily ooid grainstones that were deposited in tidal bars and channels, beaches, and islands (Figs. 52 to 54). Porosity variations exist due to the presence of carbonate mud, and as a result of pore-filling cementation in clean grainstones. Because the belt of high-energy shelf carbonates of the Smackover and the updip tidal flat equivalents of the Buckner Member of the Haynesville Formation prograded to the south through time, the position and continuity of reservoir-bearing grainstone bodies in the upper part of
I
....
Walker Creek Field of southern
porous zones have been delineated by Becher and Moore ( 1 976; Fig. 55) and corresponding depositional en vironments inferred by Becher and Moore are illustrated in Figure 56. A north-south log correlation across Oaks Field, located in northern Loui siana to the southeast of Walker
measured on several wells define at least three separate reservoirs (Chimene, 1 976). Several of these
porous Smackover is made up of porous and tight zones (Fig. 57). The reservoirs, distinct lithic units stacked
the Smackover are complex. In
Arkansas, pressure gradients
Creek Field, shows that the upper
on top of and to the south of one another, thin quickly downdip and pinchout updip within Buckner anhydritic shales (Erwin et al, 1 979). Understanding the sequence and timing of diagenesis is as important as understanding depositional en vironment relationships in predicting porosity in the Smackover Formation and many other ancient carbonate sands. The Smackover exhibits "ear ly" cements that, based on their
498
Bank Margin
..
Figure 57-Stratigraphic cross section of the Smackover Format ion i n no rthern Louis iana, based on log correlations and detailed petrographic observation. Three separated porosity zones each p inch out downdip within the Smackover and updip into the overlying tidal flat deposits of the Buckner Member of the Haynesv ille Formation.
Figu re 58-Photomic rograph of Upper J u rassic Smackover oolite showing oomoldic porosity (in white) and sig n ifi cant interparticle calc ite cementation. The grains of the orig i nal sediment have been dissolved to form pore space a nd the porosity of the original sediment has been fil led with cement. The resultant porosity of 30% i s of reservoir quality, despite permeabil ity of 1 md or less. Width of photograph represents 3 m m .
crystal morphology and distribution within the rock, resemble recent fresh-water cements (Harris, 1 980) . These blocky calcite cements may cause a significant reduction in primary intergranular porosity and locally produced seals within the up per part of the Smackover. Zones of oomoldic porosity may have also formed in these same fresh-water en vironments; the characteristically high porosities (as high as 350?o) often have low associated permeabilities of only
<)s
LOG CORRELATION OF UPPER SMACKOVER USING CLASTICITY INDEX OAKS FIELD, CLAIBORNE PARISH,LA.
LONE STAR
�I J.C.CRUMP
LONE STAR #1 S.F.SALE
EN SEARCH ll2 S.F.SALE
(AFTER ERWIN,EBY,ANO WHITESIDES,1979)
a few millidarcies (Fig. 58). Although not as common in Smackover grainstones as fresh-water cements, cements that probably form ed in the marine phreatic zone are present (see Fig. 59). Gypsum and dolomite also formed in shallow brine ponds and tidal flats where dissolved salts were concentrated by evapora tion of sea water. The gypsum was converted to anhydrite during burial; the anhydrite may contain relics of ghosts of replaced grains and often
pseudomorphs after gypsum. In ancient carbonates, some of the primary porosity that was not destroyed by syndepositional fresh water and marine cements is further decreased by "late" diagenesis. Late diagenetic processes are topics of ac tive research and little agreement among carbonate petrologists. Even what constitutes "late" is a source of argument. Generally, late is meant to include diagenesis which occurs at depth and at elevated temperatures, that is in the mesogenetic zone of Choquette and Pray (1 970). But some late processes, pressure solution for example, probably start at shallow depths and proceed continually or sporadically as burial continues. Diagenesis occurring during the burial of carbonate sands includes physical compaction, chemical compaction, cementation , and dissolution. Impor tant variables involved in late diagenetic alteration are the vertical and lateral continuity of facies and diagenetic packages, the burial history, the nature of the pore fluids, early diagenetic history, geothermal history, and fluids from associated noncarbonate rocks including organic-rich sediments. Perhaps one of the most significant late diagenetic events is the develop-
Bank Margin 499
Figure 59-Photomicrograph of Upper Jurassic Smackover oolite with porosity
filled by two stages of cementation - an early continuous, isopachous rim of marine cement, followed by a pore-filling coarse blocky cemen t . The remaining porosity is shown in blue. Width of photograph represents approximately 1.5
mm.
ment of secondary porosity, which has been documented recently in the Smackover Formation by Moore and Druckman (1 980). Although in completely understood, this process results in the dissolution of carbonate grains as well as early and late car bonate cements. The resulting vuggy porosity adds to the remnant inter particle porosity and results in ooid grainstones with enhanced reservoir properties. Variations Among Ancient Examples
Although sand bodies share many of the attributes of the modern and Jurassic examples described previous ly, sand accumulations of differing age and geographic location present almost endless variety of internal detail. For example, cross-bedding typical of many sand accumulations may be absent if the original sediment was homogenized by burrowing organisms. Extremely well-sorted sand may not contain sufficient grain size variation to produce prominent cross-bedding. Cementation may either enhance or disguise primary sedimentary features. Subtle color variations may be pro duced by slight variations in grain size, grain composition, and cementa tion style (Figs. 60, 61). Other ex amples may retain little small-scale
evidence of cross-bedding, but large scale evidence of sand body deposi tion may be preserved as bed forms in outcrop (Figs . 62, 63) . Bedding features may survive extensive dolomitization, although such dolomitization may obliterate original grain textures (Figs. 64, 65). Carbonate sand bodies occur in rocks of almost every geologic period, but are particularly abundant (and prolific hydrocarbon reservoirs) in rocks of Mississippian, Permian, Jurassic, and Cretaceous age. The range of geologic time over which sand bodies form results in composi tional differences simply from the evolutionary changes which occur within the groups of organisms pro ducing carbonate sand . For example, crinoidal sands are characteristic of the Mississippian, and molluscan or foraminiferal sands are typical of many Cretaceous sand bodies (Figs. 66 to 68); the reverse occurrences, Mississippian molluscan sands and Cretaceous crinoidal or foraminiferal sands, are extremely rare. Such evolu tionary faunal changes and their ef fects on carbonate sediments are sum marized by Wilson (1975). Secondary porosity in carbonate sand bodies is common, as ex emplified by Figure 58, but primary porosity is also often preserved at
Figure 50-0utcrop of oolite within the Cambrian Carrara Formation of Nevada. Rock exhibits small scale cross-bedding enhanced by color variations due to variable grain size and composition. Scale is in 10 em intervals_
considerable depths (Figs. 69, 70) . Often evidence of early, meteoric water diagenesis is apparent in these porous rocks (Figs. 7 1 , 72), and early diagenesis is frequently thought to be a factor in porosity preservation (Becher and Moore, 1976). Despite the compositional and diagenetic variations among sand bodies of differing ages, their recogni tion is relatively straightforward bas-
500
Bank Margin
F i gure 51-Photomicrograph of Cam
br i an ooid and skeletal grainstone from the outcrop shown in Fi g ure 60. The thin
section is a rtif i cia lly stained to reveal that the interparticle porosity is filled
one generation of iron-rich cement (blue). The grains are not stained blue because they are i ro n poor. Width of
with
-
pr1otograph represents 3 mm.
Figure 62-0utcrop photograph of Mississippian oolite exposed in Ken
tucky Individual bed sets are easily recognized, although t he detailed cross bedding has been obscured by thorough
cementation and weathering. The out
crop is approximately 5 m high.
ed on paleogeographic position geometry, bedding, and textural characteristics described previously. SUMMARY
Bank-margin sand bodies are characteristic of carbonate shelves and platforms and occur at a position just bankward of reefs; they may ex tend seaward to the bank margin or break in slope if reefs are absent.
They are typically composed of ooid and skeletal sands which form sand bodies parallel-to-flow if tidal cur rents are high ( > 100 em/sec) but normal-to-flow if currents are low (about 50 em/sec). Gaps between bank-margin islands form significant sand accumulations as tidal deltas. Sand bodies also form on structural rises j ust seaward of bank margins or well into the basin center depending on the depth of the basin and
topographic relief on the structural feature. The predominant influences on sand production and the develop ment of carbonate sand bodies are: ( 1 ) antecedent topography; (2) physical processes - tidal, wind, and wave-generated currents; (3) sea-level history; and (4) diagenesis. These in fluences do not act independently, but rather are interrelated and form a mosaic of facies at the surface which are even more complicated in three dimensions . Nevertheless, some generalizations appear possible from the studies of modern Bahamian sand bodies. Sand bodies typically occur in shallowing, upward sequences with lagoonal deposits below, and supratidal, beach, or eolian deposits above. On windward margins, sand may migrate bankward, forming extensive sand sheets on the platform. On leeward margins, sands may build seaward, burying reefs and transporting sand to deeper water beyond the shelf margins. Active portions of sand-shoal com plexes are typically clean, rippled, cross-bedded sand, devoid of grasses and most burrowing organisms . Less active portions of sand shoals become colonized by plants and bioturbated by organisms within a few tens of years.
Bank Margin 501
Fi\jure 63-Pholornicrograph of Mississ ippian oolite from the outcrop of Figure 62. Artifi(.;ial :>taining of the tt1in section indicates that the porosity has been filled by two generations of cement- a rim of iron-poor crystals surrounding each grain, followed by iron rich blocky crystals (blue). The earlier cement is s imi la r to the tresh-water phreatic cement shown in Figure 32. Width of photograph represents 2.5 mm.
Despite the compositional varia tions of carbonate sands due to evolutionary processes and the diagenetic alterations produced during subaerial exposure and burial of car bonate sands, ancient sand bodies are relatively easily identified from characteristic textures, sedimentary structures, bedding styles, geometry, and paleogeographic location within a spectrum of carbonate facies. ACKNOWLEDGMENTS
The authors thank the following per sons for their contributions to this article: Mahlon Ball, Tom Bay, Donald Bebout, Jeff Dravis, David Eby, Paul Enos, Richard lnden, William Meyers, Clyde Moore, Kathy Nichols, Peter Scholle, and Gene Shinn. Their material and concep tual additions to early versions of this Figure 64-0utcrop of dolomitized paper substantially improved the final Mississippian grainstone (Madison product. Gro up) in the Sawtooth Range, western SELECTED REFERENCES
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502
Bank Margin
Figure 65-Photomicrograph of
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Figure 66-Pholomicrograph of
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___
Bank Margin
503
Figure 67-Photomicrograph of Lower Cretaceous, mollusk (gastropod), peloid grainstone, Louisiana. Molluscan sands such as th is are rare in the Paleozoic, but occur commonly in Mesozoic and Tertiary carbonate rocks . Width of photograph represents 2 em.
Figure 58-Photomicrograph of Lower Cretaceous. foraminiferal, peloidal
grainstone, Texas. Photographed with polarizers crossed and gypsum plate in· serted to emphasize two generations of
cement. Width of photograph represent s 3mm.
South Florida, Part I, Holocene sedi ment accumulations of the South Florida Shelf Margin: Geol. Soc. America Mem. 147, 1 98 p. Erwin, C. R . , D. E. Eby, and V. S. Whitesides, 1979, Clasticity index; a key to correlating depositional and diagenetic environments of Smackover reservoirs, Oaks Field, Claiborne Parish, Louisiana: Trans. , Gulf Coast Assoc. Geol. Socs . , v. 24, p. 52-62. Garre tt, P., and T. P. Scoffin, 1977, Sed imentation on Bermuda's atoll rim: Miami Fla., Univ. Miami, Proc., Third Internat. Coral Reef Symp. , p. 87-96. and A. C. Hine, 1 979, Probing Bermuda's lagoons and reefs (abs.): AAPG Bull . , v. 63 , p. 455-456. H alley, R. B. , and P . M. Harris, 1979, Fresh-water cementation of a 1 ,000-year-old oolite: Jour. Sed. Petrology, v. 49, p. 969-988. et al, 1 977, Pleistocene barrier bar seaward of ooid shoal complex near Miami, Florida: AAPG Bull. , v. 6 1 , p. 5 19-526. Hanshaw, B. B., W. Back, and R. G. Deike, 197 1 , A geochemical hypothesis for dolomitization by groundwater: Econ. Geology, v. 66, p. 7 10-724. Harms, J. C., P. W. Choquette, a nd M. J. Brady, 1 978, Carbonate sand waves, Isla Mujeres, Yucatan, in W. C. Ward and A. E. Weidie, eds., Geology and hydrology of northeastern Yucatan: New Orleans Geol. Soc., p. 60-84. Harris, P. M. , 1979, Facies anatomy and diagenesis of a Bahamian ooid shoal: Miami, Fla., Univ. Miami, Sedimenta __
___
VII, Comparative Sedimentology Lab., 1 63 p. , 1 980, Freshwater cementation of Holocene and Jurassic grainstones (abs.): AAPG Bull . , v. 64, p. 719-720. R. B. H alley, and K. J. Lukas, 1 979, Endolith microborings and their preservation in Holocene-Pleistocene (Bahama-Florida) ooids: Geology, v. 7, p. 216-220. H ayes, M. 0., 1 979, Barrier island mor phology as a function of tidal and wave regime, in S. P. Leatherman, ed . , Bar-
___
__
rier islands, from the Gulf of St. Lawrence to the Gulf of Mexico: New York, Academic Press, p. 1 -28. Heckel, P. H . , 1 972, Recognition of an cient s hallow marine environments, in J. K. Rigby and W. K. Hamblin, eds., Recognition of ancient sediment en vironments: SEPM Spec. Pub. No. 16, p. 226-286. Hine, A. C., 1977, Lily Bank, Bahamas; history of an active oolite sand shoal: Jour. Sed. Petrology, v. 47, p. 1 554- 1 58 1 .
504 Bank Margin Figure 59-Photomicrograph of core slab from producing zone of West Felda oil field, Lower Cretaceous, Florida. Zone of production is in mollusk, peloid grainstone at a depth of about 3800 m. Width of photograph i:; 3.5 em.
Figure 70-Thin section photomicrograph of rock illustrated in Figure 69. Gastropod cast in center sur rounded by and enclosing preserved primary porosity. Width of photograph represents 3 mm.
409-422. 1 954, Bahamian calcareous sands: AAPG Bull., v. 38, p. 1 -95. Imbrie, J., and H. Buchanan, 1 965, Springer-Verlag Pub., p.
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Imlay, R. W., 1 943, Jurassic formations of Gulf region : AAPG Bull., v. 27, p. 1407- 1 53 3 .
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Land, L. S., 1 967, carbonates: Jour. and A. C. Neumann, 1 977, Shallow carbonate-bank-margin growth and structure, Little Bahama Bank, Bahamas: AAPG Bull., v. 61 , p.
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Bank Margin
505
Figure 71-Photomicrograph of green algal grainstone of Permian age, New Mexico. ''Stalactitic" or gravitational ce· rnent below grains is evidence of meteoric-water cementation in the vadose zone. Note excellent preserva tion of primary porosity (blue). Width of photograph represents 3 mm.
Figure 72-Photomlcrograph of mollusk grainstone of Cretaceous age, Florida. Fringing calcite cement is typical of ear· ly, meteoric. phreatic cements. Note ex· cellent preservation of pri mary poro�ity (blue). Width of photograph represents
3 mm.
Plate-forme (Platform Carbonate Deposits): Bull. Centre Rech. Pau SNPA, v. 7, p. 1 77-192. Lareau, J . P . , and B. H . Purser, 1 973, Dis tribution and ultrastructure of Holocene ooids in the Persian Gulf, in B . H. Purser, ed., The Persian Gulf, Holocene carbonate sedimentation and diagenesis in a shallow epicontinental sea: New York, Springer-Verlag Pub ., p. 279-328. McKee, E. D., and R. C. Gutschick, 1 969, History of Redwall Limestone of nor thern Arizona: Geol. Soc. America Mem. 1 14, 726 p. Milliman, J. D., 1974, Marine carbonates: New York, Springer-Verlag Pub . , 375 p. Moore, C. H . , and Y. Druckman, 1980, Burial diagenesis and porosity evolu tion, Upper Jurassic Smackover, Arkansas and Louisiana: La. State Univ., Dept. Geology, Applied Car bonate Research Program Tech. Ser. Cont. No. 5, 79 p. Newell, N.D., 1955, Bahama platforms, in The crust of the earth, a symposium: Geol. Soc. America Spec. Paper 62, p. 303-3 1 5. and J. K. Rigby, 1957, G eological studies on the Great Bahama Bank; in R. LeBlanc and J. G . Breeding, eds. , R egional aspects of carbonate sedimen tation: SEPM Spec . Pub. 5, p. 1 5-72. Newkirk, T. F., 1 97 1 , Possible future petroleum potential of Jurassic, western Gulf Basin , in Future petroleum pro vinces of the U.S., their geology and potential: AAPG Mem. 15, p. 927-953 . __
Off, T. , 1963, Rhythmic linear sand bodies caused by tidal currents: AAPG Bull . , v . 47, p . 324-341 . Ottmann, R. D., P . L . Keyes, and M . A. Ziegler, 1973 , Jay Field-a Jurassic stratigraphic trap: Trans., Gulf Coast Assoc. Geol. Socs., v. 23, p. 1 46-1 57. P almer, M . S., 1979, Holocene facies geometry of the leeward bank margin, Tongue of the Ocean, Bahamas: Miami, Fla. , Univ. Miami, M.S. thesis, 1 99 p. Perkins, R. D. , 1977, Quaternary sedimen-
tation in south Florida, part 2, deposi tional framework of Pleistocene rocks in south Florida: Geol. Soc. America Mem. 147, 1 98 p. and P. Enos, 1 968, Hurricane Betsy in the Florida-Bahamas area; geologic effects and comparison with Hurricane Donna: Jour. Geology, v. 76, p. 7 1 0-717. Pfiel, R. W., and J. F. Read, 1 980, Cam brian carbonate platform margin facies, Shady Dolomite, southwestern Virginia: U.S.A.: Jour. Sed. Petrology, v. 50, p.
___
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91-116. Plummer, L . N . , e t al, 1 976, Hydrogeo chemistry of Bermuda; a case history of ground water diagenesis of biocalcaren ites: Geol. Soc. America Bull. , v. 87, p. 1 301 - 1 3 1 6. Purdy, E. G . , 1 96 1 , Bahamian oolite shoals, in J. A. Peterson and J. C. Os mond, eds., Geochemistry of sandstone bodies: AAPG Spec. Vol. , 232 p. ___, 1 963 , Recent calcium carbonate facies of the Great Bahama Bank; petrography and reaction groups, sedimentary facies: Jour. Geology, v. 71, p. 334-355, 472-497. ___, 1 968, Carbonate diagenesis; an en vironmental survey: Geologica Romana, v. 7, p. 1 83-228. Purser, B. H . , 1 973, Sedimentation around bathymetric highs in the southern Per sian Gulf, in B. H. Purser, ed., The Persian Gulf, Holocene carbonate sedimentation and diagenesis in a shallow epicontinental sea: New York, Springer-Verlag Pub . , p. 157-177. ___ and G. Evans, 1 973, Regional sedimentation along the Trucial Coast, southeastern Persian Gulf, in B. H . Purser, ed., The Persian Gulf, Holocene carbonate sedimentation and
diagenesis in a shallow epicontinental sea: New York, Springer-Verlag Pub., p. 211-231. Rodriguez, R. W . , 1 979, Origin, evolu tion and morphology of the shoal, Escollo de Arenas, Viegras, Puerto Rico, and its potential as a sand source: Chapel Hill, Univ. N.C . , M.S. thesis, 71 p. Rowland, S. M . , 1 978, Environmental stratigraphy of the lower member of the Poleta Formation (Lower Cam brian), Esmeralda County, Nevada: Santa Cruz, Univ. Calif., Ph .D. dissert. Runnells, D. D . , 1 969, Diagenesis, chem ical sediments and the mixing of natural waters: Jour. Sed. Petrology, v. 39, p. 1 1 88-1 201 . Scholle, P. A., ed . , 1 978, A color illus trated guide to carbonate rock consti tuents, textures, cements, and porosities: AAPG Mem. 27, 241 p. Shaw, A. B., 1 964, Time in stratigraphy: New York, McGraw-Hill Pub., 365 p. Shinn, E . A., 1 969, Submarine lithifica tion of Holocene carbonate sediments in the Persian Gulf: Sedimentology, v. 12, p. 1 09-144. ___, 1975, Polygonal cement sutures from the Holocene; a clue to recogni-
tion of submarine diagenesis (abs.): AAPG Ann. Mtg. Abs., v. 2, p. 68. Southard, J. B., and D. J. Stanley, 1976, Shelf-break processes and sedimenta tion, in D . J. Stanley and D. J. P. Swift, eds., Marine sediment transport and environmental management: John Wiley and Sons Pub . , p. 351 -377. Swift, D. J. P . , 1972, Holocene evolution of the shelf surface, central and southern Atlantic Shelf of North America, in D. J. P. Swift, D. B. Duane, and 0. H. Pilkey, eds., Shelf sediment transport; process and pat tern: Stroudsburg, Pa . , Dowden, Hut chinson, and Ross, p. 499-574. Wilson, J. L . , 1975, Carbonate facies in geologic history: New York, Springer Verlag Pub., 469 p. Winland, H. D., and R. K. Matthews, 1 974, Origin and significance of grapestone, Bahama Islands: Jour. Sed. Petrology, v. 44, p. 921 -927. Young, G. M . , and D. G. F. Long, 1 977, Carbonate sedimentation in a Late Precambrian shelf sea, Victoria Island, Canadian Arctic Archipelago: Jour. Sed. Petrology, v. 47, p. 943-955.
508
Fore-reef Slope
F
ore-reef slope deposits have been singled out from the broad spectrum of car bonate slope deposits (Cook and Mullins, 1 982) because of distinctive characteristics associated with reef-dominated shelf margins and the proven economic importance of reef and perireef deposits . The skeletal framework and rapid vertical growth potential of ecologic reefs (Dunham , 1 970) commonly , but not invariably, produce high-relief margins with steep slopes (Fig. 1). The shallow-water tropical setting and constructional framework of reefs favor rapid syndepositional sub marine cementation that commonly extends into the fore-reef environ ment . Tropical settings and the typical highly developed ecologic niche structure of reefs favors intense bioerosion as well as framework con struction, sediment production, and binding . The products of all these reef-related phenomena leave their imprint on perireefal sediments. In an excellent summary of car bonate slope deposits, Mcllreath and James (1978) categorized carbonate margins as by-pass and depositional margins. By-pass margins are general ly steep slapes with a submarine escarpment of various origins across which sediment is transported from shallow to deep water without signifi-
Figure 1-Fore-reef slope deposits (left, slope is depositional) adjacent to Upper Devonian reefs (massive beds in the center) at Windjana Gorge, Na p ier Range, Canning basin, Western Australia. The bedded interval at the right is back-reef deposits. Height of the exposure is about 1 00 m (photograph courtesy of Phillip E. Playford}.
cant deposition on portions of the slope . Depositional margins are gentler, accreting slopes which merge gradually with basin floors. In either type of margin, the shallow-water portion may be formed by a reef or by carbonate-sand shoals (Mcllreath and James, 1978). This emphasizes that reef margins may be either steep deposits occurred as the major reef escarpments or relatively gentle slopes forming organism waxed and waned and that, conversely, not all steep (Heckel, 1 974; Wilson, 1 975; James , shelf margins are the result of reef 1 978), resulting in significant periods growth. As with all classifications, it when only mounds or banks (non does not emphasize the abundance of framework) existed or no reefs at all transitions between reef and non-reef were formed (James, 1 978, 1 982) . margins which may be repeated in Sediments of fore-reef slopes are a time and space . Modern margins pre mixture of two end members, whether sent many examples of reef growth characterized by transport process or interspersed with carbonate sand by origin of the particles. The major shoals or even with terrigenous clastic transport-process categories are directly gravity-induced deposits deposits. Indeed, many reef tracts consist of interspersed lenticular pods (resulting from talus fall, slump, and rather than continuous reef walls, for sediment gravity flow) and "gravity defying" suspension sediments. The example modern Florida reefs (Enos, particles may originate from shallow 1 977b), and mid-Cretaceous reefs of Mexico (Griffith, Pitcher, and Rice, water carbonate environments, 1 969). Other transitions are produced notably the reefs , or from elsewhere, by changes from windward to leeward primarily within the water column locations, by sheltering effects of in (pelagic), but also as deeper water termittent islands or barriers benthos and from non-carbonate (Ginsburg and Shinn, 1 964}, by sources. For lack of better: terms, the substrate variations, by sea-level fluc shallow-water carbonate environments tuations, and by the many other fac will be referred to as neritic with the tors which govern reef growth. View remaining environments described as ed from the larger perspective of oceanic or basinal sediments, follow geologic time, variations in slope ing Laporte (1979). The groupings
Fore-reef Slope
509
Figure 2-Sketch aepicting four promo n
tories and three reentrants along about
300 m horizontally of a near-ve rtica l by pass s lope in Jamaica. Promontories typical ly can be traced upward into
zones of dense coral cover at the c rest. extending downward and merging with
the fore-reef slope. Tal us cones ac coarse apices. Height oi the near-vertical portion is about 50 m (from Land and Moore,
cumulate in reentrants, with debris characteristic on ly of 1 977).
resulting from these different perspec tives are similar but not identical . Suspension sediments include car bonate mud derived from adjacent shelves, and gravity-induced deposits include pelagic sediment reworked from higher on the slope. Non carbonate sediment may also arrive by either set of processes. Definition
Fore-reef slope deposits include the spectrum of slope deposits located seaward of reef-rimmed shelves, plat forms or atolls . They will here be
considered to include all sediments with a significant input of gravity induced neritic deposits, recognizing that the limit of such deposits may not be coincident with the limit of ap preciable slope; they may extend beyond or stop short of the toe of the slope. In other words, the topo graphic slope may not be exclusively controlled by the accretion of a wedge of neritic sediment. Moreover , the seaward extent of the slope is ex tremely difficult to define in ancient examples and may even be somewhat arbitrary in modern examples where
the slope merges gradually with basin floor. The "significant input" qualifier excludes essentially oceanic or basinal deposits with a few car bonate turbidites of shallow-water origin . Diagnostic Characteristics
Slope deposits are characterized by a wide spectrum of lithologies repre senting transported reef and shelf debris mixed and interbedded with basinal-mud deposits. Talus blocks, slump deposits , debris-flow deposits, and turbidites all contribute. Perhaps
510
Fore-reef Slope
Grand Cayman Island
Belize
J amaica
Bahamas
Discovery Bay
Twin Rocks
250M
Figure 3-Representative profi!es of reefal margins and upper tore-ree t slopes from the Caribbea n . Arrows in· dicate the top of the fore-reef deposits at the base of a by-pass slope. Ve r tical exaggeration is 1 . 75X (after Rigby and Robe rts, 1 976; Jcunes and
1 979).
Ginsburg,
Figure 4-Talus cone at a reentrant in the fore-reef wal l , D iscovery Bay , Jamaica (profile in Fig. 3). Wat e r depth is 1 1 0 m. The rod-shaped sponge g row ing on the wall is approximately 50 em high (from Land and Moore, 1 977). Photoaraoh bv Noel P. James.
Fore-reef Slope
51 1
Figure 5-Biock about 5 m in height fa l len or sl umped from the fore-reef wal l , Tobacco Cay, Belize (profi le i n Fig. 3), 1 30 m depth (from James and Ginsb u rg , 1 979). Figure 6-Shelf and fore-reef slope reflection profiles from a depositional slope, south F l o r ida shelf margin. Note vertical exagg eration; steepest slopes are 1 0°. Promontories on profiles A, D, and E are drowned H o l ocene reefs. The subsurface reflector is Pleistocene rock, a lso a reefal shelf marg in, slightly modified by erosion. Pleistocene is ex posed below 20 m i n profi les A and C. "R" on profiles marks the position of a l iv i ng shelf-edge reef (after Enos, 1 977b).
20r
�-
VERTICAL 40X
0
FT
0
Mt
5000
F LO R t D A
N
t
E ..
,,
�-
----- - -
-- ____...____. _ _
V:· ' '
' I
v
the most diagnostic deposits are brec cia sheets containing blocks of reef rock; the presence of actual reef blocks is critical to distinguishing fore-reef slope deposits from other types of slope deposits. Features in dicative of slope are: synsedimentary folds formed by slump or creep; low angle truncation surfaces, probably representing slump scars; and geo petal sediment deposits in primary voids, showing that enclosing bedding was deposited at an angle to horizontal .
SETTING
The location of fore-reef slopes is, by definition, dependent on the loca tion of reefs, and the nature of the slope is directly influenced by the set ting of the associated reef. The con trols on reef location are discussed by James (1982). The global controls are oceanic temperature and circulation. With the steep latitudinal temperature gradients of the Quaternary , reefs are nearly confined to the tropics and oc cur preferentially on the western sides of oceans because of west-directed wind and current patterns in the tropics. Local controls that may be
important in predicting potential reservoir location are local circulation patterns, tectonic setting, subsidence rates and patterns , locations of suitable substrates, pre-existing con structional or erosional topography, and input and distribution patterns of terrigenous sediment . The most extensively studied modern fore-reef slopes are on steep reef-bound margins of oceanic or in tracratonic basins; some of these coincide with plate boundaries or tec tonic zones . Examples are northern Jamaica (Goreau and Goreau, 1 973; Goreau and Land, 1 974; Moore, Graham, and Land, 1976; Land and
512
Fore-reef Slope
Fig u re ?-Seismic reflection pro fil es from fore-reef slopes southwest Florida ( Uchupi and Emery, 1 967; Enos, 1 977b).
:=igure 8- .)iagrarn:-Tta�ic rt7Pt esePt�tion cf reiB.n,e ?.ecjimen 1 comr: burior;s t:y c.Jlf fe ren; tran�ort proet?sse� or. fOHHe-ef ca rbor.ate slopes w:tf! v.:=� r yl n � clst�nc� ;rom a reei-bc und basin marg,n. Suspen sior sedinwnt:; {tan} < u e sub:;lividec by origi'l. l ndividuai accumul�tio� rate;s !nd 'estlltant type:; r)f deposit sra too poorly known n n d too variable 'r'l di!fe•�m! :;oct ::ng� to �ttempt �o quantif�· them . Tolu5 devosits are C'J rl fi;·e<J to the top ot the slopP.; turbidite� typ,call)' ex1end nany kilcnotor� i :1to ���� llas:n. f.. logarithm!<� distwlc� Rt:a1e wou'd put J!e at.: r.:J mula t�c v:olllmes ;:J reag :. ;tlde� th� c ..1rves) into bo>1t�r persp.ectlvo
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Fore-reef Slope
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fiy ..H:J :tf,. ··Man ix ' ' vf iur c-:- r eei TaiiJS . ?-sorc hec com� of n1'..Jdd1 ,'-fattmedo (-;�ree1J a!g<�i; sar.c 1n cor al ta·us D:sc�ry Bay, � arn�i<;.a: 1 35 m dep'h .
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514
Fore-reef Slope
Figure 1 0- l ntraformational truncation s u rface. Bone Spri n g s limest one , basinal equivalent of the Yeso reef (Pe r · mian, Leonard) Guadalupe Mountains, Texas. Base of the su rface is at the observer ' s head; unde rlying and overly ing beds are essentially horizon tal (photog raph courtesy of Peter A. Scholle).
Moore, 1 977; Land, 1 979, Land and Moore, 1 980) , Belize ( James and Ginsburg, 1 979), portions of the Bahama Banks (Mullins and Neumann , 1 979; Schlager and Cher mak, 1 979), and Grand Cayman (Hannah and Moore, 1 979). Unfor tunately, no modern studies of slope sediments of atolls have been under taken since the pioneering work of Emery, Tracy, and Ladd ( 1 954). Studies of modern gentle depositional slopes are virtually non-existent; some data is available from study of adja cent shelves in south F1orida
(Ginsburg, 1 956; Enos, 1 977b) and the Persian Gulf (Purser, 1 973). The modern slopes of northern Jamaica, Belize, Bahama Banks (eastern Little Bahama Bank and Tongue of the Ocean) , and Grand Cayman are all characterized by a very steep by-pass zone flanked by a steep depositional slope (Figs. 2, 3) which merges seaward with a basin floor. Above the by-pass zone is the active reef with its spur-and-groove zone and steeper " fore-reef escarp ment" (Jamaica; Moore et al, 1 976) or "step" (Belize; James and Ginsburg , 1 979). The reef is common ly bounded by a sand-covered slope ("fore-reef slope" of Moore et a!, 1 976) which steepens to a "brow" (Belize) or "drop-off" (Jamaica) at the top of the by-pass slope, termed the "wall" (Belize) or "deep fore reef" (Jamaica). Sill reefs above the
by-pass slope locally trap shallow water sediment or funnel it into can yons or corrugations in the by-pass slope (Goreau and Land, 1 974) . The steepest part of the margin is a by-pass slope or wall, with slopes of 30° to near vertical or locally overhanging (Fig. 3). This configura tion may extend from as shallow as 40 m to depths of 1 60 m . This zone supports some coral, calcisponge, and algal growth and receives a veneer of mud or skeletal sand, predominantly plates of the green alga, Halimeda. Small polished grooves within the generally corrugated front denote paths of sediment transport (Moore et al, 1976). Although virtually all sedi ment by-passes this zone, slow accre tion does occur from skeletal growth and in situ cementation of skeletons and sediment . To what extent the topographic
Fore-reef Slope
515
Figure 1 1 -Al lochthonous block of reef l i mestone in well-bedded slope deposits near McSherry' s Gap, Canning basin, Western Australia (photograph cou rtesy of Phillip E. Playford).
features of these shallower, steeper parts o f the margin are relicts of the rapid glacially-related fluctuations in sea level , and are thus likely to be atypical of much of the geologic record, is an open question. No an cient example of a by-pass slope or wall has been described (Schlager and Chermak, 1 979). They likely exist in the middle Cretaceous of Mexico (Enos, l 974b), the Triassic of the Dolomites (Bosellini and Rossi, 1 974), and elsewhere, but critical exposures or definitive studies are lacking. The depositional slope below the wall is capped by a talus pile of reef derived blocks (up to 1 0 m in diameter) with slopes up to 45° . Talus forms small cones in the cor rugations of the wall (Figs. 2, 4). The talus zone is typically narrow with an abrupt lower boundary where it is overlapped by finer-grained sediment of the proximal slope. The proximal slope is as steep as 30° , but becomes gradually gentler and muddier until it merges, imperceptibly in some cases, with the basin floor or abyssal plain . The surface of the proximal slope is generally smooth (Fig. 3; Tobacco Cay , Discovery Bay) , but it may be cut by canyons (Jamaica; Land, 1 979) or riddled by small gullies (Tongue of the Ocean, Bahamas; Schlager and Chermak, 1 979). These features signal
that, although the proximal slope is accreting, much sediment is being bypassed to the lower slope or basin . The gullies provide a line source of sediment resulting in laterally con tinuous sediment belts (Schlager and Chermak, 1 979) , whereas canyons suggest point sources . No sediment cones or submarine fan complexes, the expectable result of canyon point sources, have been described from either modern settings or the stratigraphic record. The slope may also be strewn with blocks (Fig. 5), some large enough (tens of meters) to act as local sediment dams (James and Ginsburg , 1 979). Small escarp ments o f slump and/or fault origin may also trap or dam sediment (Land, 1 979). The seaward slope of the south Florida carbonate shelf with its inter mittent shelf-edge reefs is a modern example of a gentle depositional slope. Local gradients are not more than 1 0° and overall slope is about 1 o (Enos, 1 977b). Relief is confined to the upper 35 m of the slope where linear buttresses of drowned Holocene reefs rise as much as 25 m above the general slope (Fig. 6). Modern coral growth has also en crusted exposed Pleistocene (reef) rock on the steepest portions of the slope , probably wave-cut nips formed
during the Holocene rise in sea level (Fig. 6A, C) . Around areas of active reef growth, the shallow slope is strewn with coral rubble, mostly cylindrical sticks of branching Acropora cervicornis, although the bulk of reef rubble is transported landward (Ball, Shinn, and Stockman, 1 967; Enos, 1 977b). Most of the slope is a smooth accretionary wedge of muddy skeletal sediment . The slope is interrupted in two areas by extensive terraces (Fig. 7A, B, D) of outcropping Miocene(?) karstic limestone and phosphorite (Uchupi and Emery, 1 967; Burnett and Gomberg , 1 974; Enos, 1 977b). Elsewhere post-Miocene sediment has draped over these terraces forming a smooth slope to the floor of the Straits of Florida (Fig . 7C) . Such relatively gentle depositional slopes are probably more typical of slopes of reef-fringed intracratonic basins and epeiric seas which com prise so much of the stratigraphic record . It is unfortunate that modern examples have not received more detailed study. RELATION TO LATERAL FACIES
Lateral facies relationships of fore reef carbonate slopes are as varied as the reef settings which produce them.
516
Fore-reef Slope
hgure 1 2A- Orecci� Ut:'� ul f.Jrobat�e dcbri3·11o:;v crigin. L nrge hlocks of � l ime�tv�e floo.ting ir. a fine·gt<:JU l� t ·na1ri x , r1t:ar Dingo G�. 1-.J.:;�p��r Range. Cann r.g bas.r.. '1"�-..ta :n t\t:ntr::tliu
(photograph courtesy flh. lip E . Play:ord).
F1g,r� 1 2B · 9 rP. ()..�i<} rwds of pr<Jbi11'JI� detns-flvw :>�igin Thick de hriil !::e rl i n the Soyatal t=orm.atiOr'l \Lpper Creta:..,eou!:) c-.lmprising blocks afld 'Cioclasts ·rem •ime- .;.o w i v·al�1( a ; ;<.: somewhat O!dor rm1d-C(�;:a,CBous) ' e.=or':l that fom-: ltle esca(o�e ni •n the tJa<:ksroLnd. �� ·::� Ooc�� 0l-€f€12u u. Mex.-<:o .
Fore-reef Slope
517
rioure
1 3-�ns ::�eos wi�li cnannel rorm lro1n \he �: Dc.:c !or (r : : id· C!!:lt�ocous) fore· reef s:opes, Ouere�aro. Mel': ico. A. {top; Outcrop vi�"' c,f apprux· ':mate s�r ik� sec:t:on, 2 km trom tne r�t ascarpn�ent: ,;pper .:)t0mincnt ledges a:1(1 lower �edge (amYY'J) a re t>r€ccls bed,; ...... i�h dis1inct ch<.t!!nei fcrms 1 ru n· c ated b)' a sha�p faul�ed fold ;LJs' to th�; left of th� IH� Io o! view. The 'edge at 1 owe� left Is pr Obi.ibl-y i:l C' :<:wne: forrn too. ou� Is :ntensel'l' datormed. B. {mic· die) Diagram tTo atic ·"€CO!'Si r0ction o:· ttte tJ r ��.;is be-."'''l me;.su r�d sac tium;. Uppr.r fHYifllgomal<"� d bees &� te at 4� verti-;;a: e�aggerstl:;!l, iower be
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Fore-reef Slope
Figure 1 4 -Base of breccia bed in lower slope facies at Dingo Gap, Napier Range, Canning basin, Western Australia (photog raph cou rtesy Ph i l l ip E . Playford).
In areas marginal to the tropics, such south Florida and the Bahamas, active reefs are interspersed with shelf-edge skeletal-sand or oolite shoals with their various slope topographies and sedimentary facies (Mullins and Neumann, 1 979). The south Florida shelf passes laterally (northward) along both sides of the Florida plateau into areas of virtual non-deposition with a veneer of shells and even relict ooids from lower stands of sea level (Gould and Stewart, 1 955; Enos, 1 974a). Exposed non-tectonic areas of the tropics and sub-tropics, such as the Florida and Yucatan peninsulas, are characterized by low-relief karst sur faces of young (Tertiary and Quater nary) limestones with little surface drainage. The lack of relief and sur face drainage means that little detritus is delivered to the shelf. Where some terrigenous detritus is supplied by long-shore drift, as along the Florida coast, Holocene drowning of the coastline serves to confine detritus to the shoreline and inner shelf. The carbonate platforms are nearly devoid of submarine canyons, negating canyons as pathways for sediment to the slopes and basins. As a result, shelves and slopes in areas of active carbonate production receive as
nearly pure carbonate sediments, whereas areas where carbonate pro duction is lacking are starved of sedi ment. The Great Barrier Reef and the Belize barrier reef pass laterally (southward) into areas of terrigenous sediment input from hinterlands with considerable relief. The transition zones are characterized by terrigenous sediment in the shore zone and lagoons and by carbonate sediment at the shelf margin and on the slopes. Typical atolls are completely sur rounded by fore-reef carbonate slopes. LITHOFACIES AND SEDIMENTARY FACIES
The simplest scheme of fore-reef facies distribution from modern ex amples is that proposed by Emery and others ( 1 954) for atolls of the northern Marshall Islands in the cen tral Pacific Ocean. They found essen tially depth-dependent boundaries with "coral sand " containing some coarser blocks (neritic sediments) in areas less than 550 m (300 fathoms) deep, grading into " Globogerina ooze'' at depths down to 2500 m ( 1 400 fathoms). Globogerina ooze (oceanic sediment) around the atoll
pedestals passed into red clay below the carbonate compensation depth at 3600 to 5300 m (2000 to 2900 fathoms). Recent work with advanced techniques on steep fore-reef slopes has considerably refined the nature of the "coral sand" and emphasized the controls of biotic productivity and fore-reef topography on sediment types. Studies of ancient slope se quences have confirmed and further refined observations on sedimentary facies of modern slopes. Each of the two major transport process categories mentioned above includes several important members with contrasting sediment characteris tics. Gravity-induced processes in clude talus fall, slump, grain flow, debris flow, and turbidity currents. These processes tend to produce discrete deposits with some distinguishing characteristics. In con trast, suspension sediments all settle particle-by-particle, but because of varied sources, produce a spectrum of intergradational sediment types. Pelagic biogenic products and mud derived from shallow water are the important carbonate sources, but these may be diluted or overwhelmed by terrigenous mud, volcanogenic sediment, siliceous biogenic products, or basinal evaporites. Figure 8 shows
Fore-reef Slope
519
Figure 1 5-Top of 15 m thick channel form breccia bed (Fig. 1 3). El Doctor
(mid-Cretaceous) fore-reef slope,
Queretaro, Mexico. Tne top % m of the bed (at the hammer) is graded; large
c lasts from the ungraded massive part of the bed project up into the graded portion (just left of the hammer). The overiylng thin beds are pelagic !ime mudstone and chert.
qualitatively the spatial relationships of the major process-related sediment types . Deposits of Gravity-induced Processes
Gravity-induced processes and the resultant deposits have been analyzed and described by Dott ( 1 963), Cook and others ( 1 972), and Middleton and Hampton ( 1 973) . Because these ex cellent references on process and recognition of deposits are available,
attention will be focused on criteria related to the fore-reef setting in the following discussion. Talus is confined to a narrow zone at the top of the slope in all modern examples (James and Ginsburg, 1 979; Moore et al, 1 976; Mullins and Neumann, 1 979; Schlager and Cher mak, 1 979) . Talus is mentioned in many ancient sequences (Playford, 1 980; Tyrrell, 1 969) but no detailed accounts have been published. Blocks
are skeletal particles and fragments of the reef or peri-reefal sediment bound by submarine cement or encrusting organisms. Rocks which form the substrate of the reef and rocks or particles from back-reef environments may also be included . Thus, lithologies and cements produced in widely varying settings may be represented. Skeletal particles vary with geologic age of the deposit as the dominant reef-forming and bind-
520
Fore-reef Slope
Fig u re 1 6-Graded brecc ia bed, dipping 90°, from El Doctor (mid-Cretaceo us) fore-reef slope, Puerto de O rtiga, H idalgo, Mex ico. Top of the bed ( rig ht} g rades into pelagic lime mudstone .
Figure 1 7-Debris bed showing reverse grad i ng and l arge clasts proj ecti ng i nto the overlying fine-grained roc ks , D i ngo Gap, Napie r Range, Can n i ng basin, Western Australia (photograph courtesy of Phillip E. Playford).
ing organisms varied. Arrangement of the blocks is chaotic, sorting is non existent, and matrix is confined to mud and sand filtered into cavities . Such sediment commonly forms perched cones or layered geopetal fill (Fig. 9). Talus deposits may form linear prisms along the reef margin or
may be isolated cones at reentrants (Figs. 2, 4). Slumps may originate in the reef or reef-wall, but their deposits would be distinguished from talus only if other slope deposits were incorporated, or isolated "exotic blocks" were deposited. Slumps may originate
anywhere on the slope (Fig . 8), but their formation is favored by steep slopes , rapid deposition, fine-grained sediment , and lack of intergranular support such as interlocking, bound, or cemented particles . The first two conditions are best met on the upper slope, but the last two are commonly confined to the lower slope. Slump scars may be represented by concave up, low-angle "intraforma tional truncation surfaces, " (Cook and Enos, 1 977a) spectacularly represented in many fore-reef settings such as the Permo-Pennsylvanian of the Sverdrup Basin, Arctic Canada (Davies, 1 977), the Triassic Steinplatte reef of Austria (Wilson, 1 969), the Mississippian Lodgepole Formation of Montana (Smith, 1 977), and the Permian Bone Spring Limestone of the Capitan escarpment , Texas-New Mexico (Fig. 10; Wilson, 1 969). Whether these features result from slumps or some other type o f submarine erosion (Yurewicz, 1 977), or both, is still uncertain as suggested by Wilson's early designation as "cut and fill or slumping" (Wilson, 1 969) . Slump deposits range from coherent but discordant blocks to
Fore-reef Slope
521
hgv r e 1 8-l)ef!cate S U I VtVUIH Of dttO':S iiOW transport A. (left) In tact c.us!er:� or radiolitid:'l (center, upper left), a reef form ing rudist bivalve. from me sha!low El Abra (mid.C�etaceous) fore·ree� slope deposits near El Lono, Q:Jeretaro, Mex ico. Radioli !ids form rather conosivc clusters through interleaving shell fla nges. B (bElow) Platy clast of
laminated i i me mudstone. sandwiched bet•Neen two ir:tervals of fine, grainy
limestone (outlined by dots where obscure). Tt1e grainy irrl ervals were pro· bably iigtJ tly cemented d u r ing lra11sport to rorm a brit�le clast. Note the more robust neig hbofs above and below, E' Doctor (equ ivalent ro El .A.ora) fore-reef, San Joaquin . O'J eretaro, Mexico.
masses highly contorted by soft sediment folds or faults . By defini tion, slumping does not include loss of internal cohesion resulting in a fluid matrix (Dott, 1 963). Examples of discordant exotic blocks are rudist reef blocks in the basin-margin Tamabra Limestone of the mid Cretaceous of northeastern Mexico (Carrasco, 1 977; Enos, in press), algal-reef blocks as large as 1 km in the Lower Devonian Nubrigyn For mation of southeastern Australia (Conaghan et al, 1 976), blocks up to 100 m across from the Devonian of the Canning basin, Western Australia (Fig. 1 1 ; Playford and Lowry, 1 966; Playford, 1 980) , the "Cipit Limestone" of the Triassic in the Dolomites, northern Italy (Bosellini and Rossi, 1 974), and the Permian Bell Canyon Formation of West Texas and New Mexico (Rigby, 1 958). The "haystacks" of the North Jamaica island slope (Moore et al, 1 976) may be modern examples; these are large, rounded, lithified blocks found at depths below 230 m subsea . Similar blocks litter the Belize fore reef slope (Fig . 5; James and Ginsburg, 1 979). Examples of folded slump deposits are described from the Permian Bell Canyon and Cherry Canyon Formations, west Texas and New Mexico, by Rigby ( 1 958).
Lithologically, fore-reef slump deposits represent the entire spectrum from actual reef blocks to pelagic limestone or shale. Because slumps maintain internal cohesion, mixing of lithologies is largely confined to in terlayering of the original deposits or
piling up of successive allochthonous slump masses . Their distinguishing characteristics are discordance in at titude and, commonly, contrast in lithology with the surrounding rocks. Internal deformation may be slight to extreme. Adjacent beds may be
522
Fore-reef Slope
Figure 20-Steeply inclined clasts
(above and tu right uf harnmer head; also at lower right) ncar lateral mmgin
of
channel-form breccia bed (Fig. 1 38).
Figure 1 9-Randomly oriented clasts in breccia f rom Golden La ne fore- reef. Co re of Tamabra Formation (mid· C retaceous), Poza Rica field, Veracruz, Mexico. Core width is 9 em.
distorted by drag or "bulldozing" as the slump mass is emplaced. Superja cent beds drape or onlap the slump mass (Fig . 1 1). Grain-flow deposits have been ten tatively identified from modern fore reef deposits of the Little Bahama Bank (Mullins and Neumann, 1 979), and from Upper Devonian foreslope breccias adjacent to carbonate "buildups" in Alberta, Canada (Hopkins, 1 977). The modern deposits, described from a single cored example, are inversely graded, contain floating clasts, and have in jection structures at the base, features which Middleton and Hampton ( 1 973) speculated might occur in
grain-flow deposits. Fore-reef slopes are one of the few settings where the conditions for grain flow - slopes near the angle of repose and a supply of granular particles (Lowe, 1 976) - may be maintained , but con siderable doubt still exists as to whether grains flow is a significant depositional mechanism. Debris flows form breccia beds that are the most spectacular elements of many fore-reef slope deposits (Fig. 1 2) . They have been given various labels ranging from purely descriptive to highly genetic: breccia beds, megabreccia sheets, debris sheets, rudite sheets , olistostromes, mass breccia flows, submarine mass flows, debris avalanches, debris flow [deposits] (Mcllreath and James, 1 978). Ancient examples of fore-reef breccias of probable debris-flow origin include the Nubrigyn Forma tion (Lower Devonian), New South Wales, Australia (Conaghan et al, 1 976); Napier Formation (Middle and Upper Devonian), Canning basin, Western Australia (Playford, 1 980); Perdrix and Mount Hawk Formations (Upper Devonian), Alberta Canada, (Cook et al, 1 972). Hare Fiord For-
mation (Middle Pennsylvanian-Lower Permian) Sverdrup Basin, Arctic Canada (Davies , 1 977), Bone Spring Limestone (Leonardian, Permian), Delaware basin, Texas-New Mexico (Pray and Stehli, 1 962); Cherry Canyon and Bell Canyon Formations (Guadalupian, Permian), Delaware basin , Texas (Rigby, 1 958), and Tamabra Limestone (mid Cretaceous), northeast Mexico (Car rasco, 1 977; Enos, 1 974b, 1 977c). Few modern examples have been described, in part because of the dif ficulty of coring such materials, but Crevello ( 1 978; Crevello and Schlager, 1 980) reports a debris sheet that extends for 6400 sq km beneath the floor of Exuma Sound, a deep (2000 m) reentrant in Great Bahama Bank. Debris beds range from less than one to several tens of meters thick and may extend several tens of kilometers from the basin margin . It is rarely possible to reconstruct the deposi tional slope angle, but some beds were apparently deposited on slopes of no more than 1 ° (Cook et al, 1 972). It is even more difficult to determine whether ancient flows moved onto the flat basin floor, but
Fore-reef Slope
523
Figure 2 1 -Breccia beds from core of Tamabra Formation . mid Cretaceous, fore-reel ot Golden Lane " atol l , " Poza nica field, Ve rac ruz, Mexico (from Enos, 1 977c) A. (left) Rudist +ragment lime g rainstone and microfossil lime wackestone c i<J s ts l n matrix of pelagic
l i me m uds, numerous stylol ites. B . (right) Sparse matrix a nd sutu red clasts i n cluding c h e r t (dark clast lett center); in c l i ned fabric probably imb rication. Cores are 9 t;fll wide.
Playford ( 1 980) reports debris sheets that extend "several hundred meters over adjoining basin deposits." The modern debris sheet in Exuma Sound, Bahamas, extends perhaps 1 50 km beneath the basin floor (Crevello , 1 978; Crevello and Schlager, 1 980) . Most debris beds are sheets in three dimensions, but lenticular to distinct ly channelled forms occur interlayered in the same sequences (Fig. 1 3 ; Enos, 1 973; Playford, 1 980). Lower bound aries of debris beds are sharp and typically planar (Fig. 1 4). Sole marks are very rare ; load casts are the most common. Underlying beds are com monly undeformed, but are locally deformed by shear fractures or dragfolds, or eroded by channels
(Fig . 1 3) . Sharp upper contacts are the norm (Fig . 1 5) , but some sheets terminate by grading into pelagic deposits (Fig . 1 6) . Hummocky tops formed by large projecting blocks are characteristic of debris-flow deposits (Fig . 17), but planar upper surfaces are more common (Fig . 1 5) . Lateral and frontal margins are steep with abrupt termination or thinning . Internally, sorting is extremely poor with mud and small clasts inter spersed with blocks as large as 200 m in maximum dimension (Figs. 12 and 14; Conaghan et al, 1 976; Playford, 1 980). Despite such contrasts in size, delicate fossils and platy clasts are preserved (Fig. 1 8) . Graded beds, both normal (Figs. 1 5 and 1 6) and reverse (Fig. 1 7) , are reported but are not common . Particle orientation is commonly random ("chaotic ," Fig. 1 9), but orientation parallel to sheet boundaries occurs near the base (Fig . 1 6) or less commonly at the steep sides or the top (Enos, l 977a). These are the regions of maximum flow whereas the interior of a flow may be rafted as a rigid plug (Hampton , 1 972) . Domains of parallel "packed" clasts within flows have orientations from bedding-parallel to nearly ver tical (Fig . 20) . Imbricate packing of clasts is also observed, and Hubert, Suchecki, and Callahan (1 977) documented an inclined fabric shin gled downslope, the reverse of normal imbrication, in the Cow Head Breccia (Cambro-Ordovician) of western Newfoundland. Matrix content of breccia beds is characteristically high enough that clasts appear to float in matrix (Figs. 18 and 21A; "floatstone" of Embry and Klovan, 1 973). Indeed the ac cepted definition of debris flow is support of clasts by matrix strength
during transport (cf. Middleton and Hampton, 1 973). However, clast sup port is also common (Fig . 16, 2 1 B , "rudstone" of Embry and Klovan, 1 97 1 ) . This need not imply that matrix support was lacking during transport, although some breccias that apparently had very little original matrix have been interpreted as debris-flow deposits. Apparent matrix volume can be reduced considerably by dewatering , as lime muds typically have about 751J!o original porosity (Enos and Sawatsky, 1 98 1 ) , somewhat higher than partially con solidated (dewatered) muddy in traclasts and at least 350Jo greater than partially lithified grainy clasts. Matrix is also susceptible to preferen-
524
Fore-reef Slope
Figure 22- Matrlx with pelagic microfossi ls (mostly calcispheres with some g l obular foraminifers) surrounding lightly cemented clast of shal low-water skeletal l imestone ; note thin rim of ce ment on projecting particles at right, Tamabra Formation (mid-Cretaceous) fore-reef of Golden Lane "atol l . " Poza Rica field, Veracruz, Mexico. Scale bar is 5 m m (fr�m Enos. 1 977c)_ ,
tial removal during stylolite forma tion (Fig. 21). Matrix comprises lime mud (silt and clay size) or a mixture of com positions , depending on slope and/or peri-reefal sediment composition . The common occurrence of pelagic fossils in the matrix (Fig. 22) points to deeper slope environments as a major source of matrix mud, either by ero sion during transport or incorpora tion in the original flow , for example through slumping. Composition of the clasts reflects the source and is the best means of
establishing a reef source , indepen dent of reconstructing regional geology. Clasts typically comprise blocks from : the reef, reef talus, or peri-reefal deposits; individual bioclasts or other carbonate grains; and muddy intraclasts. The percent age of muddy intraclasts, which are predominantly slope derived increases basinward (Enos, 1 973, and in press) . Grainy blocks must be bound or par tially cemented to survive transport (Fig. 22) and reflect the organic bind ing or submarine cementation that is so common in reefs and upper fore reef slopes. Blocks and bioclasts vary from angular (Figs. 14, 1 5 , 22) to rounded (Figs. 16 and 2 1 A) , depend ing on degree of early lithification, original shape , and pre-transport history. Muddy intraclasts vary in composition depending on the com position of slope deposits and their locus of origin. Burrowed, or laminated lime mud and chert are
common lithologies (Fig. 2 1 ) . Although typical muddy clasts were probably unlithified, they are com monly angular or even platy (Figs. 1 5 , 1 8 , 2 1 B). This is because they tend to separate along bedding planes and are protected from abrasion by matrix support during transport. Deposits of turbidity currents, tur bidites or allodapic limestones ("allodapische Kalke," Meischner, 1 964) , extend further into the basin than other types of gravity-induced deposits; they may be spread many kilometers across essentially flat basin floors . They can provide the most remote indication of the proximity of reefs, and, by paleocurrent reconstruction, point to the direction of the reef source. The difficulty lies in distinguishing reef-derived from non-reef carbonate sources. Again, the only clue to reef origin may be the composition of the clasts, unless regional paleogeography points to a
Fore-reef Slope
reef source or other, more obvious, fore.-reef deposits (above) are in terbedded . If the clasts arc large eno ugh fo include lithoclasts , the diagnosis is considerably easier, although reef origin should not be in terpreted based on limaU fragment� alone. If on!� bioclasts are present 1 as is likely to be the case in more distal and therefore more critical areas, the task becomes one of reconstructing a typical reef assemblage for the par ticular age in question. In other respects , fore-reef carbonate tur bidites do not differ from other car bonate turbidites . They are character ized by graded beds, partial or com plete Bouma sequences (Fig . 23), sharp or erosive basal contacts, repetitious interbedding of grainy deposits containing shallow-water biota in pelagic deposits, and lack of shallow-water sedimentary structures (oscillation ripples, large-scale cross bedding) or biogenic structures. Sole
marks are rarely observt!d iu car bonate turbidites, in contra�t ro ter rigeno us turbidites. \Vhether this simply rctlccts relative scarcity of
bedding-plane t:xpusures i n carbonate
525
Figure 23-Detrital limestone bed show· ing Bouma divisions A-g raded {note peb bles at base), B-planar laminated (?), and C-cross-laminated (with oversteepened r i pple forms). The interval above the rip ple forms may be a contin uation of the same depositional unit with coarse-line layering In division D, overlain by pelagic lime mudstone with c11e11 (black, division E). From El Doctor (mid-Cretaceous) fore-reef, Pu�rta d� Oniga, H idalgo, Mex.ico (from Enos. 1 97 4b).
turbidites, particularly where the pelagic interbeds are also predom inantly carb()natt:, _or whether it reflects differences in rates of erosion between carbonate and terrigenous muds or even differences in character istics of carbonate turbidite flow is at material (Bosellini and Rossi, 1 974). present an unanswered question (lves , Studies of modern settings have 1 953) . documented progressive down-slope increase in mud content mixed with Suspension Deposits skeletal sands with various transport Suspension deposits adjacent to histories (James and Ginsburg, 1 979; reefal or carbonate-sand shelf Moore et al, 1 976), the potential con margins are mixtures of shelf-derived tribution of carbonate-shelf mud to carbonate mud with pelagic car surrounding basins (Neumann and bonates ("peri-platform ooze ," Land, 1 975), and the composition of Schlager and James, 1 978) and, com some of the particles (Moore et a!, monly, with suspension material of 1 976; James and Ginsburg, 1 979). other origins such as biogenic silica, The general characteristics and terrigenous mud, or pyroclastic microfacies of ancient deep-water car-
526
Fore-reef Slope
Figure 25-Suspension rlAposits A. (left) lam i n ated lime mudstone with sparse burrows filled with grainy carbonate reworked from interbedded fine-grained turbidites. shallow El Abra (mid· Cretaceou�) rore-reet deposits, near El L obo Queretaro, Mex1co. B. (bel owi Beddi ng-plane exposure of burrows shown in section A.
Figure 24-Finely laminated pelag ic l i me mudstone overlain by bioclastic-brecc ia bed of probable debris-flow orig i n , El Abra (m id-C retaceous) fore-reef deposits near El Lobo, Queretaro, Mexico. Slab, 7
em wide.
bonates have been treated in some detail by Wilson ( 1 969, 1 975), and Cook and Enos ( 1 977b) , Carbonate mudstone, generally dark-colored, with fine lamination (Figs. 10, 1 5 , 24), thick, structureless beds, or bioturbation (Fig. 25) are typical lithologies . Spiculitic (siliceous) limestone and nodular or interbedded chert are other common lithologies. The biota that dominates modern pelagic carbonates - plank tonic foraminifers, pteropods, and calcareous nannofossils - first ap peared in the Mesozoic, so older pelagic carbonates must be distinguished by other forms such as nautiloids, ammonoids, graptolites, planktonic crinoids, calcispheres (at least in the Mesozoic) , tintinnids, and radiolarians . Few criteria in peri-plat form oozes distinguish the contribution of reefs or shelf-derived mud, although modern sediments offer a few leads which warrant further investigation. Distinctive, silt-sized, faceted chips are produced abundantly by boring activity of endolithic sponges, notably
Cliona (Futterer, 1 974; Moore et al,
1 976). Such particles, although not confined to them, are probably most abundant in tropical, shallow-water reefs (Futterer, 1 974; Moore et al, 1 976). Moore and others ( 1 976) sug gested that sponge chips with algal borings ("micrite envelopes") may be useful shallow-water indicators . Distinctive silt-sized skeletal particles such as Homotrema (a red, encrusting foraminifer), tunicate spicules (a hemicordate), certain Penicillus needles (green alga; Perkins, McKenzie, and Blackwelder , 1 972), and Halimeda plate fragments (green alga) are also of a size and density readily transported in suspension and , thus, are possible clues to tropical-shelf,
but not exclusively reefal , sediment contributions. Schlager and Chermak ( 1 979) noted that high organic con tent and a green tint characterize sedi ment from Bahaman deep-water basins that was deposited during relatively high stands of sea level and thus received shelf-sediment contribu tions. Low-stand sediment, predominantly pelagic, is white with little organic matter . The problem common to all such fine-grained tracers is preservation through the diagenetic curtain. The shallow-water skeletal components mentioned consist of metastable car bonate phases, aragonite and magne sian calcite, and are thus very suscep tible to alteration during stabilization
Fore-reef Slope
527
f:gure 2G-Zoop.h}·c�.o<: trace ��s.ls. A. (�bove) e,ot-,;t�d p�lag:c "!'licrofossi: lirne u n ; d�ton�'rro;n Tom�t ra l:rnestona (mid-Crel�ceo�}. f�re-n�ef of tt:o? G
Pc2a Rica !rend, 'v'eracru<., M e:>::ico . Fed is shown in Fiuur� 3 1 : wit..ftl': Dh i
numerous ZooPiwcustraces (a1 rom;). Klamogun Formaticn {Mi
528
Fore-reef Slope
DEPOSITIONAL MARGIN
Reef- Derived Block
SHALLOW WATER REEF
Massive Calcarenites
Shal low Water
Derived Li.ne Sreccia
Slope-Derived Lime ·
Pelagic/Hemipelcrgi£ -Umestone
Breccia Hemipelagic lima Mudstone,
with �ritrbfbrinotional
Truncation Surface "
Mi){ed Slope and Morgin· · Deriwd Sre«ia
Graded Coltarenit�
Intraformational T runcotion Surfaces
May
Occur Anywhere A long Slop e
P I a tf o r m
l
o-�
4 • -. .. � --
_ � _ _ _ _ _ _
Margin
Profile
Slop e --------f-ool- Basin
--
Figure 27-Siope r1odels from Mcll reath
and James (1 978). A. (above) Schematic model for a shallow-water, reef
domi nated, depositional carbonate
marg i n , and ill ustration of a hypothetical
sequence of deposits from slope accre tion. B. (facin g page) Schemat:c mode l for a shal low-water. reef dominated, by pass type of carbonate marg i n .
ed by down-slope creep, synsedimen tary boudins (Hubert et al , 1 977), and intraformational truncation surface (Fig . 1 0) , discussed with slump deposits. STRATIGRAPHIC SEQUENCE
to calcite. In contrast, pelagic car bonate particles are composed of calcite, except for aragonitic pteropods. If recognizable textures were destroyed, a tenuous clue to shallow-water input might remain in the form of abnormally high Mg or Sr (from aragonite) contents. The paucity of bedding-plane ex posures in carbonates, noted above, precludes observation of many types of biogenic structures potentially diagnostic of slope or basinal en vironments (Fig . 25B) . Such struc tures as can be identified in vertical section , notably Zoophycus, are local ly well-represented in carbonate rocks ranging in age from at least Devonian to Tertiary (Fig. 26) . Sedimentary structures indicative of depositional slope in suspension deposits are soft-sediment folds form-
The foregoing discussion illustrates the extreme variability of fore-reef slope deposits. For example, particle size ranges from nannofossils to the largest sedimentary particles known, with blocks exceeding 1 km in max imum dimension. This range of as much as 9 orders of magnitude in linear dimensions may occur within a single depositional unit. Although chaos seems the rule, the spatial distribution of sedimentary types shown in Figure 8 suggests that pro gradation of a fore-reef slope should produce a recognizable facies succes sion, despite extreme interdigitation. Suspension deposits should be suc ceeded by a preponderance of car bonate turbidites , then debris-flow deposits, slump deposits, possibly grain-flow deposits, reef talus, and finally the reef itself. Mcllreath and
James ( 1 978) present schematic models of slope sequences showing these general trends with repeated in terbeds of lime mud ("pelagic/ hemipelagic limestone' ') reproduced here as Figure 27; Figure 28 is a diagrammatic representation of an ac tual prograding shelf margin and a transgressed margin from the Triassic of the Dolomites, northern Italy. Distinguishing fore-reef slopes from slopes adjacent to carbonate sand margins is best accomplished using the petrology of the transported blocks which present large samples of the shelf margin . Indeed, many deposits eventually recognized as transported blocks from slopes were originally interpreted as patch reefs or bioherms (Pray and Stehli, 1 962; Conaghan et al, 1 976; summary by Mountjoy et al, 1 972, Table 1). The composition of bioclasts may also be used to reconstruct a reef assemblage. James ( 1 972, 1 982) outlines major contributors to framework reefs at various times and emphasizes the times in which no framework reefs were formed, and thus no fore-reef slopes are to be expected. A more
Fore-reef Slope
529
Breccia
BY-PASS MARGIN SHALLOW WATER REEF
Massive Calcarenites Breccia
Intraformational Truncation Surfaces May Occur Anywhere Along Slope Profile Platform Margin
..
,
Slope �-o� - r----- Debris Apron ----�
=
-�==����!!� Figure 28-Accretionary profiles of car· bonate bank margins from the Dolomites, South Tyrol, Italy, as inter· p reted from the fore-reef slope deposits. Left, an expanding, steadily prograding bank marg in. Right, a bank margin that prog raded and then shrank wh ile conti· nuing to accrete vertically. Green is massive dolomite, red is fore-reef talus (right) and exotic blocks (lett), and blue is basinal pyroclastic rocks (after P. Leonardi, from Wilson, 1975).
detailed discussion is presented by Heckel ( 1 974). In a general way, it is predictable that the rigid framework of reefs resulting from organic binding and syndepositional cementation , would lead to a dominance of lithoclasts or blocks in fore-reef slope sequences. In contrast, the prevalence of detrital carbonate sands at a shelf margin should lead to a slope sequence dominated by calcarenite beds ,
especially turbidites. While the geologic record seems to support this assertion, quantitative comparisons are lacking. It must also be noted that carbonate sands can be cemented by submarine mechanisms or during periodic exposure to fresh water, and therefore contribute copious blocks to the adjacent slope. Blocks alone do not indicate proximity to a reef; the petrology must be carefully examined.
LOG RESPONSE
Reconstruction of a low-relief fore reef slope in the Permian of west Texas from log correlations is il lustrated in Figure 29. Log expression of a wedge of fore-reef deposits within basinal limestones at the margin of a steep escarpment in the Poza Rica field, Mexico , is shown in Figure 30. Some of the prominent mechanical log markers were cor-
530
Fore-reef Slope
EAST
WEST
SAI>I ANDIIU
Seo Level
___
GUIR1E't4
sn L••1
'TOf" Of CLEAII fORK GROUP
"TuBB' IIULLW.OCON
� - tooo'
- 1 000'
:i
VALE SHAO..I!:
LUEO ERS FV. VALERA
-zooo·
--- -2000'
--- - 3000'
-4000'
0
Legend:
ELECTRIC AIII O R,_OIOACTIVE LOG5 OE$1GNATEO
}
P • SE:LF 6
•
-
POTENTIAL
GAMMA
ftAT
R • RESISTIIIITY N � NEUlRON
Fig u re 29- Low- rel ief reefal shelf margin
"Z" s howi ng shelf, slope, and basinal profile. Oiamond- M field, Scu rry County, Texas. Vertical exag geration about 4X (from Van Siclen. 1 958).
related throughout the Poza Rica field by Barnetche and Illing ( 1 956). The markers represent intervals of finer grained and non-porous rocks, in part suspension deposits, within a thick sequence of rudist grainstones and breccias, subsequently interpreted as sediment gravity-flow deposits derived from the adjacent Golden Lane reefs (see case histories to follow; Enos, 1 977c). ECONOMIC CONSIDERATIONS
Some of the factors contributing to the vast economic potential of reefs are shared directly by the uppermost part of the fore-reef slope. The original texture of many reefs, coarse skeletal grains with framework sup port, may be drastically modified by
diagenesis, beginning with submarine cementation. The setting of typical reefs as topographic highs at shelf margins makes them particularly susceptible to exposure to fresh water which may enhance economic poten tial by enlarging and connecting pores or may reduce porosity by cementa tion. Dolomitization also seems par ticularly prevalent in reefs . Finally, settings at the edge of basins, wherein most petroleum is generated, favors charging of reservoirs. These considerations apply directly, although in reduced measure, to the uppermost fore-reef slope. Most of the particles are derived directly from the reef and deposited with framework support as, for example, talus blocks. Submarine cementation is particularly prevalent in upper fore reef settings in modern reefs of Jamica (Land and Moore, 1 977), Belize (James and Ginsburg, 1 979), Cretaceous, Mexico (Aguayo , 1 978) ,
and Devonian , Arctic Canada (Davies and Krouse, 1 975). The depths to which submarine cementation is most active are at present uncertain. Land and Moore ( 1 977) suggested that cementation by magnesian calcite in the Jamaican fore-reef is confined to the warm, mixed surface layer above the top of the thermocline at about 1 00 m . Evidence from Belize (James and Ginsburg, 1 979) indicates active aragonite and Mg-calcite cementation to at least 1 75 m subsea and Mg calcite precipitation to at least 600 m , somewhat above the base of the local thermocline layer. Later work in Jamaica (Land and Moore, 1 980) also indicates important cementation and/or alteration within the ther mocline layer . Definitive statements must await further study, particularly within and below the thermocline layer, but present indicatiom are that the generally deleterious effects of submarine cementation are most pro-
Fore-reef Slope
531
sw
M t: l f R S SLJB S E A
2000------����
0
I OOOm
�O R I ZONTAL
S CALE
L E G E N D
C a l'ltpoct
� <>c.e. t<> No�' ron
Som·CompactJ
01 1-Wct
.. C��tG�!
c u:, c o o = Q
�'f'��:�OR test•
T
/
/ ...-
r
I
wm
RefiQbf�} Oo•Jbtful Cu• relcii i.?IIS
� o�v Lottr
'C, a
APll < OJ..
Olt-Wot•r c�r.tccl
Figur� �-Aepre�enta1lve cross section of Poza Rica fieod, Veracruz:, Mexico, !';howlng gene(al ct:afac�er o� mec!'lan:cal·1og · 'hOr izon s . · · �ome cf W11 i ch ara pe!agie wac�flstones wlmln fore;-eef debris (irom Barnetche and llling, • 956).
532
Fore-reef Slope
'JS� HEllMAN) TilES
0
20 Km 10Mi
0
\ ® 0
�-
O i l F I E LDS
F\
EST{J/110��\0\
MEX I C O
..,.;J, A IIJN
Figure 3 1 -location of th e Poza Rica trend (red) and the adjacent Golden lane fieids, Veracruz, Mexico. The green area rimmed by oil fields 1s the m id · Cretaceous Go den lane "atol l . " Line A·B is location of Figure 33; s h ort line at Cerro Azu l is location of Fig u re 32 (after Guzma n , 1 967).
nounced in the surface mixed layer, significant within the thermocline layer, and much reduced below. It must be remembered, however, that submarine cementation does occur in the deep sea (Milliman, 1 974). Pleistocene fluctuations of sea level have exposed major portions of the fore-reef slope to subaerial diagenesis in both the vadose and phreatic zones (Land and Moore, 1 977). Similar fluctuations have occurred in some past eras, for example, the Permian (Dunham, 1 972) and Cretaceous (Ward , 1 979). Deeper fore-reef slopes may in directly share the early diagenetic tendency toward change of shallower zones in proportion to the amount of
sediment derived from the reef and upper slope. In general, however, the prevalence of finer-grained sediment and reduced water temperatures and circulation mean lower initial porosities and less diagenetic altera tion . Moreover, the mineralogy of post-Jurassic planktonic carbonate coccoliths and foraminifers is stable calcite, in contrast to the preponderantly metastable shallow water skeletons. The deeper fore-reef is not devoid of economic potential , however, as case histories to follow will indicate. In fact , there might be more case histories if long-standing practice did not terminate exploration at the shelf edge. Historically, much of the economic interest in slope
deposits has focused on their use as "proximity indicators, " pointing the way to nearby shelf margins. The mineral potential of fore-reef slope deposits is also related to a high potential for porosity development in associated facies plus the probability of hydrothermal mineralization because of the association of reefs with continental margins and oceanic volcanic pedestals (atolls). Important hydrothermal copper- , silver-, and lead-sulfide deposits occur in basinal limestones and reef-derived breccias in the Zimapan area, Hidalgo , Mex ico (P. Enos, personal observations) but no causal association with sedimentary facies has been demonstrated.
Fore-reel Slope
533
TYPICAL SEISMOLOG ICAL SECTtON MORALILlO - CERRO · AZUt (OlD GOLDEN ·
Figure 32-Typical seismic section from escarpment of G o ld en Lane (right) to a Tamabra Fo rm at i o n field, Moralillo, mid· Cretaceous, Veracruz, Mexico. Section location is shown in Fig u re 31 (from Guzman, 1 967).
2,500
METERS
�00
Figure 33-Regional c ross section of the Tampico embayment showing mid Cretaceous limestone formations and the locatio n of the Poza Rica field . Acuatempa is a Golden Lane field. Pro duction at Poza Rica is confined to the northeast l imb of the gentle anticl i ne . I n i tia l slopes at the basin m a rgi n cannot be reconstructed ; present dips on the Golden Lane escarpment, a s steep 30°, are essentially depositional slope. Cross section location shown in Figure 3 1 (from Enos, 1 9 77c)
CASE HISTORIES OF PETROLEUM PRODUCTION
The best known example of fore reef petroleum production is from the Poza Rica trend, Veracruz, Mexico (Barnetche and Illing, 1 956; Enos, 1 977c, and in press) . The Poza Rica trend is a series of oil and gas fields located 10 to 20 km basinward of t he subsurface escarpment of the mid Cretaceous Golden Lane "atoll" (Figs. 3 1 and 32) . Estimated ultimate recovery is more than two billion bar rels , most of it from the greater Poza Rica field. Reservoir rock is the Tamabra Limestone, a wedge of detrital carbonate that thins from the base of the Golden Lane escarpment and interfingers with basinal lime mudstone and wackestone of the Up per Tamaulipas Formation (Fig . 33). Rudist-fragment lime packstone and
grainstone (Fig. 34), probably from sediment gravity flows, are the most important reservoir lithologies. Polymict breccias (Fig. 2 1 ) , apparently debris-flow deposits, resulting
SW
0
POZA
RICA
10
fiJ Upper Cretaceous
2 0 Km
lower Cretaceous
(g) Tomoulipos
and dolomite are locally productive. Porosity includes some primary in tergranular pores, but secondary molds of rudist skeletons are the most important type (Fig . 35). Deep down dip circulation of fresh water from the exposed Golden Lane escarpment with its well-developed Cretaceous cave system is suggested as the pro cess producing the extensive secon dary porosity (Enos, 1 977d). This in dicates that even distal fore-reef zones may share in porosity enhancement resulting from the exposure o f elevated reef complexes to an exten sive freshwater system Trapping is provided by pinchout of the reservoir facies into dense basinal limestone coincident with draping in a gentle anticline (Figs . 33 and 30). Seals are provided by non porous Upper Cretaceous pelagic lime mudstones overlying the Tamabra reservoir. Source rocks are uncertain but are thought to be Upper Jurassic shales rather than the adjacent Cretaceous basinal limestones which are low in organic content (Enos, in .
ACUA-
TEMPA
II @
Coast
NE
Tamobro Abro
press) . Preliminary reports from the giant new Reforma trend in southern Mex ico cite fore-reef deposits and talus as major contributors to the reservoirs (Fig. 36; Flores, 1 978; Santiago and Mejia, 1 980; Viniegra, 1 98 1 ) . Salt flowage structures appear to control the accumulation sites in this highly dolomitized reservoir, however. In the offshore trend of the adjacent Campeche shelf, even more prolific production is obtained from car bonate breccias of Paleocene age, likewise ascribed to fore-reef deposi tion , although few details are as yet available (Santiago and Mejia, 1 980; Viniegra, 1 98 1 ) . Some of the production at the Golden Spike field, Upper Devonian, Alberta, Canada, is from coarse skeletal debris interbedded with dark lime muds as much as 1 km basin ward from the reef margin . This is at tributed to debris flows from the reef margin (Mountjoy et al, 1 972). Many other examples of production or potential production from fore-reef
534
Fore-reef Slope
Figure 34-Ruoist-tragment g rains one . Tamabra Formation . P rimary in ·
tergranular porosity ot 3207� was reduc· ed lo 2 % by cementation. Secondary moldlc porosity 1 3 .5o/o . Widtt� ot core slab is 1 0 em (from Enos, 1 977c).
slope facies can be expected with in creased recognition of this facies and its economic potential.
SELECTED REFERENCES
C . , 1978, Sedimentary environ ments and diagenesis of a Cretaceous reef complex, eastern Mexico : An. Cen tro Cienc. del Mar y. Limnol , Univ.
Aguayo,
.
Nacional Auton Mexico, v. 5, no. 1 , p . 83- 140. Ball, M. M., E. A. Shinn, and K. W. Stockman, 1 967, Geologic effects of Hurricane Donna in south Florida: Jour. Geology, v. 75, no. S, p. 583-597. Barnetche, A . , and L. V. llling, 1956, The Tamabra Limestone of the Poza Rica oil field, Veracruz, Mexico: Mexico D .F . , 20th Internat. Geol. Cong . , 38 p . Bosellini, A., and D . Rossi, 1974, Tri assic carbonate buildups of the
dolomites, northern Italy, in L. F. Laporte, ed . , Reefs in time and space: SEPM Spec. Pub. 1 8 , p. 209-233 . Burnett , W . C . , and D . N. Gomberg, 1974, Uranium in phosphate deposits from the Pourtales Terrace, Straits of Florida (abs.): Geol. Soc. America Abs. with Programs, v. 6, no . 7, p. 1 028- 1 029. Carrasco V . , B . , 1977, Albian sedimenta tion of submarine autochthonous and allochthonous carbonates, east edge of
Fore-reef Slope
Figure 35-Thin section photo mic rog raph of Tamabra Formation rud ist-frag ment g rainstone in plane lig h t . Pn mary intergran u lar porosity (red) and intragranular porosity (g reen) a re reduc ed by cE:ltriE;l l l l Moldic IJOI OSity (blue), also reduced by cement, lies within rhe preserved m icrite ri nd. Note that all types of porosity contain residua' oil stai n . Scale bar (lo�er righ!} is 0.5 mm.
water carbonate environments: SEPM Spec. Pub. 25, 336 p. and H. T. Mullins, 1982, Carbon ate slopes and basin margins, in P. A. Scholle, D . G. Bebout and C . H . Moore, eds . , Carbonate depositional environments: AAPG Mem. 33, this volume. et a!, 1 972, Allochthonous car bonate debris flows at Devonian bank (reef) margins, Alberta, Canada: Bull. Canadian Petroleum Geols., v. 20, no . 3, p. 439-497. Crevello , P. D . , 1978, Debris-flow depos its and turbidites in a modern car bonate basin, Exuma Sound, Bahamas: Coral Gables, Fla. , Univ. Miami, M.S. thesis, 133 p . and W . Schlager, 1 980, Carbonate debris sheets and turbidites, Exuma Sound, Bahamas: Jour. Sed. Petrology, v. 50, no. 4, p. l l2 1 - l l48. Davies, G. R., 1977, Turbidites, debris sheets, and truncation structures in Up per Paleozoic deep-water carbonates of the Sverdrup Basin , Arctic Archipelago, in H. E . Cook and P. Enos, eds., __
__
the Valles-San Luis Potosi platform, Mexico, in H. C. Cook and P. Enos, eds . , Deep-water carbonate en vironments: SEPM Spec . Pub. 25, p . 263-272. Conaghan, P. J . , et a!, 1976, Nubrigyn algal reefs (Devonian), eastern Australia; allochtonous blocks and mega-breccias: Geol. Soc. America Bull. , v. 87, p. 5 1 5-530. Cook , H. E . , and P. Enos, 1 977a, Deep water carbonate environments-an in troduction, in H. E. Cook and P. Enos, eds. , Deep-water carbonate en vironments: SEPM Spec. Pub. 25, p. 1-3. and , eds . , 1 977b, Deep-
__
__
__
535
Deep-water carbonate environments: SEPM Spec. Pub. 25 , p. 221 -247 . and H. R. Krouse, 1 975, Carbon and oxygen isotopic composition of late Paleozoic calcite cements, Canadian Arctic Archipelago-preliminary results and interpretation: Geol. Survey Canada, Paper 75- l B, p. 21 5-220. Dott, R . H. , Jr. , 1963, Dynamics of sub aqueous gravity depositional processes: AAPG Bull . , v. 47, p. 1 04- 128. Dunham, R . J., 1 970, Stratigraphic reefs versus ecologic reefs: AAPG Bull . , v. 54, p . 193 1 - 1 932. , 1 972, Guide for study and dis cussion of individual reinterpretation of the sedimentation and diagenesis o f the Permian Capitan geologic reef and associated rocks, New Mexico and Texas: Permian Basin Sec . , SEPM Pub. 72-1 4, 235 p. Embry, A. F., and J . E . Klovan, 1 97 1 , A Late Devonian reef trace on north eastern Banks Island, Northwest Ter ritories: Canadian Petroleum Geols. Bull. , v. 19, p. 730-781 . Emery, K . 0 . , J . I . Tracey, Jr . , and
__
__
536
Fore-reef Slope
NW
DE POSITIONAL
E u K i nic
We l l s
Ta l u s
Basin
D i st a l 61
51
Pro x i m al 81
71
fore Reef 91
E N V I RO N M E NTS
SE
Reef
Bock
" Ex t e r i or
Complex
Reef
Lagoon
101
Ill
121
"
I ntenor
" B a rrier ( R e ef ? ) "
Lag o o n
150
1 KM
L I T H OLO G Y Argillaceous
mudstone Cherty mudstone
Skeletal packstone
wackestone packstone
Broclastc beds
POROSITY
Inter·
crystalline
Breccia
Skeletal
lntercrystalline
Inter crystal line Inter granular I n tra
Boundstone.· ru dist
coral
Skeletal
packstone wa�;kHstonc
Framework supported, shelter
Breccia
clean , sorted Skeletal packstone
Inter
granular
l ntra yranulur
Exterior lagoon and barrier reef? Interpretation is not documented.
Millolid wackestone packstone
1-racture Inter
crystal
line
granular
NOTE: ENTIRE SEQUENCE
EXTENSIVELY OOLOMITIZED.
Figure 36-Schematic section showing facies inte rpretation from dolomi tized reservoir in a reefal p latform ma rgin and adjacent slope deposits, Sit io-Grande field , Reforma t rend, I sth m u s o1 Teh ua ntepec , Tabasco and Chi apa s , Mexico (after Flores . 1 978) .
H. S. Ladd, 1954, Geology of Bikini and nearby atolls: U.S. Geol. Survey Paper 260-A , 262 p . Enos, P . , 1973, Channelized submarine carbonate debris flow, Cretaceous, Mexico (abs .): AAPG Bull . , v. 57, p. 777. ___, 1 974a, Surface sediment facies of the Florida-Bahama Plateau: Geol . Soc. America, Map no . 5 , 5 p. ___ , 1974b, Reefs, platforms, and basins o f Middle Cretaceous in nor theast Mexico: AAPG Bull . , v. 58, p . 800-809. ___ , 1977a, Flow regimes in debris flow: Sedimentology, v. 24, p. 1 3 3- 142. , 1 977b, Holocene sediment accum ulations of the south Florida shelf margin, in P. Enos and R. D. Perkins, eds . , Quaternary sedimentation in south Florida: Geol . Soc. America, Mem. 1 47, p. 1 - 1 30. ___ , 1 977c, Tamabra limestone o f the Poza Rica trend, Cretaceous, Mexico, __
in H . E. Cook and P . Enos, eds . , Deep-water carbonate environments: SEPM Spc. Pub. 25 , p . 273-3 14. __ , 1 977d, Diagenesis of a giant : Poza Rica trend, Mexico (abs . ) , in D . G. Bebout and R . G . Loucks, eds . , Cretaceous carbonates o f Texas and Mexico , applications to subsurface ex ploration: Austin, Univ. Texas, Bur. Econ . Geol . , Rept . Invest . 89, p. 324. ___ , in press , Poza Rica field, Vera cruz, Mexico , in P. 0. Roehl and P . W. Choquette, eds . , Carbonate petroleum reservoirs; a casebook: New York, Springer-Verlag Pub. __ and L. H. Sawatsky, 1 9 8 1 , Pore networks in carbonate sediments: Jour. Sed. Petrology, v. 5 1 , p . 961-985. Flores, V. Q. , 1978, Paleosedimentologia en Ia zona de Sitio Grande-Sabancuy: Petroleo lnternac . , v . 36, p. 44-48. Futterer, D. K. , 1974, Significance of the boring sponge Cliona for the origin of fine grained material o f carbonate sediments: Jour. Sed. Petrology, v. 44, no. I, p . 79-84. Ginsburg, R. N . , 1956, Environmental relationships of grain size and consti tuent particles in some south Florida carbonate sediments : AAPG Bull., v. 40, no. 10, p. 2384-2427 . __ and E. A. Shinn, 1964, Distribu-
tion o f the reef-building community in Florida and the Bahamas (abs. ) : AAPG Bull . , v. 48 , p. 527. Goreau , T . F., and N . I . Goreau, 1 973, The ecology of Jamaican coral reefs I I ; geomorphology, zonation , and sedimentary phases : Bull. Marine Sci . , v. 2 3 , p . 399-464. and L. S. Land, 1974, Fore-reef morphology and depositional processes, north Jamaica, in L. F. Laporte, ed . , Reefs i n time and space: SEPM Spec. Pub. 1 8 , p. 77-89. Gould, H. R . , and R. H. Stewart, 1 955, Continental terrace sediments in the northeastern Gulf of Mexico , in 1. L . Hough and H . W . Menard, eds . , Find ing ancient shorelines: SEPM Spec. Pub . 3, p. 2- 19. Griffith , L . S., M . G. Pitcher, and G . W. Rice, 1 969, Quantitative environmental analysis of a Lower Cretaceous reef complex, in G. M. Friedman, ed . , Depositional environments i n carbonate rocks: SEPM Spec. Pub . 14, p . 1 20- 1 38. Guzman , E . J., 1967, Reef type strati graphic traps in Mexico: Proc. 7th World Petroleum Cong . , v. 2, p . 461-470. Hampton, M. A . , 1972, The role of sub aqueous debris flow in generating tur__
Fore-reef Slope
bidity currents: Jour. Sed . Petrology, v. 42, p . 775-793 . Hanna, J . C . , and C . H. Moore, 1 979, Quaternary temporal framework o f reef to basin sedimentation, Grand Cayman Island, British West Indies (abs.): San Diego , Geol. Soc. America Ann. Mtg. Heckel, P. H . , 1 974, Carbonate buildups in the geologic record; a review, in L . F. Laporte, ed., Reefs modern and ancient : SEPM Spec. Pub. 1 8, p .
90-1 54. Hopkins, J . C., 1 977, Production of fore slope breccia by differential submarine cementation and downslope displace ment of carbonate sands, Miette and Ancient Wall buildups , Devonia, Canada, in H. E. Cook and P. Enos, eds . , Deep-water carbonate en vironments: SEPM Spec . Pub. 25, p .
1 55- 1 70. Hubert , J . F., R . K . Suchecki , and R. K. M. Callahan, 1 977, The Cowhead Breccia; sedimentology of the Cambro-Ordovician continental margin, Newfoundland, in H. E. Cook and P . Enos, eds . , Deep-water carbonate en vironments: SEPM Spec. Pub. 25 , p .
125- 154. Ives, C., 1 953, The unanswered question: New York, Southern Music Pub. Co . , 8 p. James, N. P . , 1 978, Facies models 10; reefs: Geosci. Canada, v. 5 , p . 1 6-26. __ , 1 982, Reefs, in P. A. Scholle, G. Bebout and C. H. Moore, eds . , Carbonate depositional environments : AAPG Mem. 33, this volume. __ and R. N. Ginsburg, 1979, The seaward margin of Belize barrier and atoll reefs : Internat . Assoc. Sedimen tologists Spec . Pub. no . 3, 1 9 1 p . Land, L . S . , 1979, The fate of reef derived sediment on the north Jamaican slope: Marine Geology, v. 29, p. 55-7 1 . __ and C . H . Moore , Jr . , 1 977, Deep forereef and upper island slope, north Jamaica, in S . H. Frost, M . P . Weiss, and J. B. Saunders, eds . , Reefs and related carbonates-ecology and sedimentology: AAPG Stud. Geology No. 4, p. 53-65 . __ and __ , 1 980, Lithification, micritization , and syndepositional diagenesis of biolithites on the Jamaican island slope: Jour. Sed. Petrology, v . 50, p. 357-369. Laporte, L . F . , 1 979, Ancient environ ments: Englewood Cliffs, New Jersey, Prentice-Hall Pub . , 2nd ed . , 160 p. Lowe, D. R., 1976, Grain flow and grain flow deposits: Jour. Sed. Petrology , v . 46, p . 1 88-199 . Mciireath, I . A . , and N. P . James, 1978, Facies models 1 2; carbonate slopes: Geosci . Canada, v. 5, no . 4, p . 1 89-199. (also in R . G . Walker, ed . ,
1 979, Facies models: Geosci . Canada, Rept. Ser . I , p. 1 33-149). Meischner, K. D . , 1 964, Allodapische Kalke, turbidite in ri ff-nahen sedimentations-becken, in A. H . Bouma and A . Brouwer, eds . , Tur bidites: Amsterdam, Elsevier Sci . Pub . , p . 1 56- 1 9 1 . Middleton, G. V . , and M . A. Hampton, 1 973, Sediment gravity flows; mechanics of flow and deposition in G . V . Middleton and A . H . Bouma, eds . , Turbidites and deep-water sedimenta tion: Los Angeles, Calif. , SEPM Pacific Sec . , p. 1-38. Milliman , J . D., 1974, Marine carbonates: New York, Springer-Verlag Pub . , 375 p. Moore, C . H . , E. A. Graham, and L . S . Land, 1 976, Sediment transport and dispersal across the deep fore-reef and island slope ( - 55 m to - 305 m), Discovery Bay, Jamaica: Jour. Sed. Petrology, v. 46, p. 1 74- 1 87. Mountjoy, E. W . , et al, 1972, Alloch thonous carbonate debris flows worldwide indicators of reef complex, banks, or shelf margins: Montreal, 24th Internat . Geol . Cong . , Sec . 6, p. 1 72- 1 89 . Mullins, H . T. , and A . C. Neumann , 1 979, Deep carbonate bank margin structure and sedimentation in the northern Bahamas , in L. J. Doyle and D. H. Pilkey, eds . , Geology of con tinental slopes : SEPM Spec. Pub. 27, p . 1 65-1 92. Neumann, A. C . , and L . S . land, 1975, Lime mud deposition in the Bight o f Abaco, Bahamas, a budget : Jour. Sed. Petrology, v. 45, p. 763-786. Perkins, R. D . , M. D . McKenzie, and P. L. Blackwelder, 1 972, Aragonite crystals within Codiacean algae; distinc tive morphology and sedimentary im plications: Science, v. 175, p. 624-626. Playford, P. E . , 1 980, Devonian "Great Barrier Reef" of Canning basin, Western Australia: AAPG Bull . , v. 64, p. 8 14-840. __ and D. C. Lowry, 1 956, Devonian reef complexes of the Canning basin , Western Australia: Geol. Survey Western Australia, Bull. 1 1 8, 1 50 p . Pray, L . C . , and F . G . Stehli , 1 962, Allochthonous origin, Bone Springs " patch reefs , " west Texas (abs . ) : Geol. Soc. America Spec . Paper 73, p.
2 1 8-219.
Purser, B . H . , ed . , 1 973, The Persian Gulf; Holocene carbonate sedimenta tion and diagenesis in a shallow epicon tinental sea: Berlin, Springer-Verlag Pub . , 47 1 p . Rigby, J. K . , 1 958, Mass movements i n Permian rocks a t Trans-Pecos, Texas : Jour. Sed. Petrology, v. 28, p. 298- 3 1 5 .
537
and H. H. Roberts, 1976, Geology reefs and marine communities of Grand Cayman Islands, British West Indies : Brigham Young Univ., Geol. Stud. Spec . Pub. A, p. 1-95. Santiago, A . J., and D. 0. Mejia, 1980, Giant fields in the southeast o f Mexico : Trans . , Gulf Coast Assoc. Geol . Socs . , v . 30, p . 1 -3 1 . Schlager, W . , and A . Chermak, 1 979, Sediment facies of platform-basin tran sition, Tongue of the Ocean, Bahamas, in L. L. Doyle and 0. H. Pilkey, eds., Geology of continental slopes: SEPM Spec. Pub. 27, p. 193-208. __ and N. P. James, 1 978, Low magnesium calcite limestones, forming at the deep-sea floor, Tongue of the Ocean, Bahamas: Sedimentology, v. 25, p . 675-702. Smith , D. L . , 1 977, Transition from deep to shallow-water carbonates, Paine Member, Lodgepole Formation, central Montana, in H. E. Cook and P. Enos, eds . , Deep-water carbonate en vironments: SEPM Spec. Pub. 25, p . 1 87-20 1 . Tyrrell, W . W . , 1 969, Criteria useful in interpreting environments of unlike but time-equivalent carbonate units (Tansill-Capitan-Lomar) , Capitan Reef Complex, west Texas and New Mexico, in G. M . Friedman, ed . , Depositional environments in carbonate rocks: SEPM Spec. Pub. 14, p. 80-97. Uchupi, E . , and K. 0. Emery, 1 967, Structure of continental margin off Atlantic coast o f United States: AAPG Bull . , v. 5 1 , p. 223-234. Van Siclen , D. C . , 1 958, Depositional topography-examples and theory: AAPG Bull. , v. 42, p . 1 897- 1 9 1 3 . Ward, J . A . , 1979, Stratigraphy, deposi tional environments, and diagenesis o f t h e E l Doctor platform , Queretaro , Mexico: Binghamton, State Univ. of New York, Ph.D. dissert . , 1 72 p . Wilson, J . L . , 1 969, Microfacies and sedimentary structures in " deeper water" lime mudstone, in G. M. Fried man, ed . , Depositional environments in carbonate rocks: SEPM Spec. Pub. 1 4, p. 4-1 9. __ , 1975, Carbonate facies in geologic history: Berlin , Springer-Verlag Pub . , 47 1 p . Viniegra 0 . F., 1 98 1 , Great carbonate bank of Yucatan, southern Mexico: Jour. Petroleum Geology, v. 3, no . 3, p. 247-278. Yurewicz, D. A . , 1 977, Sedimentology of Mississippian basin-facies car bonates, New Mexico and west Tex as-the Rancheria Formation, in H . E . Cook and P . Enos, eds . , Deep-water carbonate environments : SEPM Spec . Pub . 25, p. 203-219. __
540
Basin Margin
7'T'
..\(
I I /
I
T
he need to expand the search for energy resources into deeper marine en vironments has intensified the importance of better understan ding the nature and origin of car bonate slope settings and of acquiring a working knowledge of their charac teristics. Coarse-grained mass-flow deposits beyond the shelf break in clastic environments are known to form major petroleum reservoirs (Barbat , 1 958), and it is likely that clastic deep-water environments will continue to be future exploration targets (Hedberg, 1 970; Curran et al, 197 1 ; Gardett, 1 97 1 ; Nagel and Parker, 1 97 1 ; Yarborough, 1 97 1 ; Schlanger and Combs, 1 975; Walker, 1978; Wilde et al, 1 978) . With the concept of plate tectonics, seismic stratigraphy, and advances in seismic reflection technology, a more sophisticated approach to understan ding the developments of continental slopes has emerged (Doyle and Pilkey, 1979). Consequently, this understanding has placed more em phasis on the geological history and petroleum potential of slope settings (for example, Hedberg , 1970; Burk and Drake, 1974; Weeks, 1 974; Bouma et al, 1976; Thompson, 1 976; Wang and McKelvey, 1 976; Bloomer, 1977; Schlee et al, 1 977; Mattick et al, 1 978). Well-documented examples of petroleum reservoirs in carbonate slope and basinal settings are fewer in number than their clastic counter parts. However , it is likely that more deep-water carbonate reservoirs will be discovered as exploration and research continue in this domain (Cook et al, 1 972; Cook, 1979a; Cook and Enos, 1 977b; Enos, 1 977a, b, in press; Scholle, 1 977; Flores, 1 978; Mullins et al, 1 978; Mullins and Neumann, 1 979; Cook and Egbert, 198 1a; Enos and Moore, this volume).
LITTLE B A i f A M A BANK
25 KrA
===
F i g u r e 1 -General ized nea r·su rface
sed imentary facies map of the deep car bonate bank margins in the northern Bahamas (modified from Mullins and N e umann, 1 979).
This paper presents a guide for recognizing carbonate slope sequences that are found farther seaward than fore-reef slopes. Fore-reef slopes are examined by Enos and Moore (1982, this volume) . Criteria are presented that we believe are common , or could be common, to all slopes regardless of whether they are adjacent to ac tive, passive, or buttressed carbonate platform margins . SUMMARY OF DIAGNOSTIC CHARACTERISTICS
As Dott and Bird (1 979) aptly state, in the modern ocean ' 'there is an enormous advantage in knowing what the environment is, but it is not always obvious what features of the modern case are preservable and which are most diagnostic of that particular environment." The ancient
record "provides a much better view of features too large to be displayed in box cores but too small to be resolved at all (or only as lines and shadows) on conventional subbottom profiles . It is self-evident , therefore, that the best sedimentary models are constructed both from outstanding modern and ancient examples . " Deep-water carbonate slope sedi ment is sufficiently different from shallow marine carbonate sediment so that confusion between the two should not pose a serious problem . However, this is not always the case with slope and basin settings, because these environments can have several attributes in common. First, as discussed by Cook and others (1972) , Wilson ( 1 975), Mcllreath and James (1978), and Enos and Moore (this volume) among others, some carbonate slopes can merge gradually with basin
Basin Margin
541
Figure 2-ldeal ized sequences of sedimentary textu res and structures in hypothetical single-mec hanism deposits of deep-water coarse clastic sed iments (modified f rom M iddleton and Hampton. 1 976).
floors, lacking a clear topographic separation. Second, both slopes and basins are sites of extensive fine grained pelagic and hemipelagic sedimentation. Third, mass transport deposits commonly form a large pro portion of slope, base-of-slope, and continental rise sequences. One of the most distinct differences between basin and slope sediment , however, is reflected in the instability of slope and base-of-slope en vironments. The disruption and reorientation of sediment into slides and chaotically deformed masses may be very common on slopes. In con
trast, the frequency and scale of these features in carbonate basins are usually relatively minor. Another important difference be tween slope and basin relates to the texture and geometry of their sedi ment gravity-flow deposits. Slope and
base-of-slope sequences commonly in clude massive-bedded conglomerate and megabreccia containing clasts more than 10 m across. These deposits, commonly interpreted to have been transported by debris flows, are either sheet-like or con fined to relatively narrow, erosional channels with little evidence of significant meandering. In contrast, sediment gravity-flow deposits in basinal settings are usually finer tex tured and can even occur in well organized thinning- and fining upward sequences similar to clastic submarine fan facies (Cook and Egbert, 198 la, b).
PHYSIOGRAPHY AND TYPES OF SLOPES
Modern carbonate platforms are common at latitudes usually less than about 30° . Sequences of relatively pure carbonate sediments are found on and around isolated oceanic islands (for example, Bermuda and the Pacific atolls), major offshore banks (for example, the Bahamas and Belize), and along peninsulas that are distant from clastic input (for exam ple, Florida and the Yucatan; Neumann, 1 977). Ginsburg and James (1974) classify modern car
bonate platforms into two broad categories: ( l ) open platforms or ramps, such as West Florida and Campeche Bank; and (2) rimmed platforms, such as the Bahama Banks, the Great Barrier Reef, south Florida and Belize. The difference be-
542
Basin Margin
Figure 3-Ciassifica tion of subaqueous sedim ent-gravity flows (from M iddleton and H ampton, 1 976).
F i g u re 4-SEM m i c rograph of soft peri· platform ooze. N ote il hund<�nc:e of aragonite needles and low-Mg c a lcite cocol iths (Emil/ana huxley!). This m i xture of shallow-wa t e r bank-derived a ragonite needles plus p l a n kton ic debris such as cocculiHls. fora m i n i fe rs and pteropods is typi cal of modern peri-platform oozes on ca rbonate slopes in tr1e Bahamas. Core 7507-020 from Tong u e of the Ocean. Depth is 1 000 fatt10ms; age is late Pleistocene o r H olocene (from S c h la g e r and James, 1 9 7 8 ; courtesy W. Sc h la g e r) .
tween these platform types is that the rimmed variety is characterized by a high energy rim of reefs, islands, or sand shoals found at the edge of the platform, whereas the former is an open, gently seaward dipping shelf devoid of any continuous energy bar rier at the platform edge (Ahr, 1 973) . Carbonate slopes surround all of these modern carbonate platforms. However, only the slopes in the northern Bahamas have been studied
in detail (Fig. 1 ; Neumann, 1 974; Neumann et al, 1 977; Mullins, 1 978; Mullins and Neumann, 1 979; Schlager and Chermak , 1 979; Mullins et al , l 980a, b ; Crevello and Schlager, 1 980; Mullins et al, 1 98 1 , in prep.). Other regions of modern carbonate slopes have received only preliminary examination (Ewing et al, 1 969; Worzel et al, 1 973; Burne, 1 974; Moore et al, 1 976; Mitchum, 1 978; Hana and Moore, 1 979; James and
Ginsburg, 1 979; Land, 1 979) . Thus, the examples from modern carbonate slopes in this paper will be strongly biased toward the northern Bahamas, as this is where most of the com prehensive studies have been com pleted. In the northern Bahamas (Fig. 1 ) , carbonate slopes are found along: ( I ) open oceans (an example being north of Little Bahama Bank) which are open to extremely large fetches; (2) open seaways, which are open to oceanic circulation at both ends (ex amples being the Straits of Florida and Providence Channels); or (3) closed seaways, which are open to ocean circulation at only one end (an example being Tongue of the Ocean; Exuma Sound). These slopes show great variability in morphology and sedimentary facies relationships that
Basin Margin
TYPE
IB
) 7 50 ::0?lt ;� ��:b ' _
----
E :t: 1a. w Cl
675'
---
·
' .- k�
7. 5 km ----+
---------] 900 , 675
4 km ---
Fig u re 5-Examples of 3.5 KHz PD R